Immunoglobulin (Ig) light chain, kappa type, variable (V) domain; The members here are composed of the immunoglobulin (Ig) light chain, kappa type, variable (V) domain. This group contains the standard Ig superfamily V-set AGFCC'C"/DEB domain topology. The basic structure of Ig molecules is a tetramer of two light chains and two heavy chains linked by disulfide bonds. There are two types of light chains: kappa and lambda, each composed of a constant domain (CL) and a variable domain (VL). There are five types of heavy chains (alpha, gamma, delta, epsilon, and mu), which determines the type of immunoglobulin formed: IgA, IgG, IgD, IgE, and IgM, respectively. In higher vertebrates, there are two types of light chain, designated kappa and lambda, which seem to be functionally identical, and can associate with any of the heavy chains.

                        10        20        30        40        50        60        70        80
                ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
1DBB_L        2 VVMTQIPLSLPVNLGDQASISCRSSQSLihsnGNTYLHWYLQKPGQSPKLLMYKVSNRFYGVPDRFSGSGSGTDFTLKIS 81
Cdd:cd04980   1 IVMTQSPASLSVSPGERVTISCKASQSI----SSNYLAWYQQKPGQAPKLLIYYASTLHSGVPSRFSGSGSGTDFTLTIS 76

                        90       100       110
                ....*....|....*....|....*....|
1DBB_L       82 RVEAEDLGIYFCSQSSHVPPTFGGGTKLEI 111
Cdd:cd04980  77 SVEPEDAAVYYCQQGYTFPYTFGGGTKLEI 106

Immunoglobulin (Ig) light chain, kappa type, variable (V) domain; The members here are composed of the immunoglobulin (Ig) light chain, kappa type, variable (V) domain. This group contains the standard Ig superfamily V-set AGFCC'C"/DEB domain topology. The basic structure of Ig molecules is a tetramer of two light chains and two heavy chains linked by disulfide bonds. There are two types of light chains: kappa and lambda, each composed of a constant domain (CL) and a variable domain (VL). There are five types of heavy chains (alpha, gamma, delta, epsilon, and mu), which determines the type of immunoglobulin formed: IgA, IgG, IgD, IgE, and IgM, respectively. In higher vertebrates, there are two types of light chain, designated kappa and lambda, which seem to be functionally identical, and can associate with any of the heavy chains.

                        10        20        30        40        50        60        70        80
                ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
1DBB_L        2 VVMTQIPLSLPVNLGDQASISCRSSQSLihsnGNTYLHWYLQKPGQSPKLLMYKVSNRFYGVPDRFSGSGSGTDFTLKIS 81
Cdd:cd04980   1 IVMTQSPASLSVSPGERVTISCKASQSI----SSNYLAWYQQKPGQAPKLLIYYASTLHSGVPSRFSGSGSGTDFTLTIS 76

                        90       100       110
                ....*....|....*....|....*....|
1DBB_L       82 RVEAEDLGIYFCSQSSHVPPTFGGGTKLEI 111
Cdd:cd04980  77 SVEPEDAAVYYCQQGYTFPYTFGGGTKLEI 106

Immunoglobulin C1-set domain;

                          10        20        30        40        50        60        70        80
                  ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
1DBB_L        120 VSIFPPSSEQLtSGGASVVCFLNNFYPKDINVKWKIDGSERQNGVlNSWTDQDSKDSTYSMSSTLTLTKDEYERHNSYTC 199
Cdd:pfam07654   1 VYVFPPSPEEL-GKPNTLTCLVTGFYPPDITVTWLKNGQEVTEGV-KTTPPSPNSDWTYQLSSYLTVTPSDWESGDEYTC 78

                  ....*..
1DBB_L        200 EATHKTS 206
Cdd:pfam07654  79 RVEHEGL 85

Immunoglobulin V-Type;

                           10        20        30        40        50        60        70        80
                   ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
1DBB_L          19 ASISCRSSQSlihSNGNTYLHWYLQKPGQSPKLLMYKVSN--RFY--GVPDRFSGSG--SGTDFTLKISRVEAEDLGIYF 92
Cdd:smart00406   2 VTLSCKFSGS---TFSSYYVSWVRQPPGKGLEWLGYIGSNgsSYYqeSYKGRFTISKdtSKNDVSLTISNLRVEDTGTYY 78

                   ...
1DBB_L          93 CSQ 95
Cdd:smart00406  79 CAV 81

Immunoglobulin (Ig) light chain, kappa type, variable (V) domain; The members here are composed of the immunoglobulin (Ig) light chain, kappa type, variable (V) domain. This group contains the standard Ig superfamily V-set AGFCC'C"/DEB domain topology. The basic structure of Ig molecules is a tetramer of two light chains and two heavy chains linked by disulfide bonds. There are two types of light chains: kappa and lambda, each composed of a constant domain (CL) and a variable domain (VL). There are five types of heavy chains (alpha, gamma, delta, epsilon, and mu), which determines the type of immunoglobulin formed: IgA, IgG, IgD, IgE, and IgM, respectively. In higher vertebrates, there are two types of light chain, designated kappa and lambda, which seem to be functionally identical, and can associate with any of the heavy chains.

                        10        20        30        40        50        60        70        80
                ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
1DBB_L        2 VVMTQIPLSLPVNLGDQASISCRSSQSLihsnGNTYLHWYLQKPGQSPKLLMYKVSNRFYGVPDRFSGSGSGTDFTLKIS 81
Cdd:cd04980   1 IVMTQSPASLSVSPGERVTISCKASQSI----SSNYLAWYQQKPGQAPKLLIYYASTLHSGVPSRFSGSGSGTDFTLTIS 76

                        90       100       110
                ....*....|....*....|....*....|
1DBB_L       82 RVEAEDLGIYFCSQSSHVPPTFGGGTKLEI 111
Cdd:cd04980  77 SVEPEDAAVYYCQQGYTFPYTFGGGTKLEI 106

Immunoglobulin (Ig) lambda light chain variable (V) domain; The members here are composed of the immunoglobulin (Ig) light chain, lambda type, variable (V) domain. The basic structure of Ig molecules is a tetramer of two light chains and two heavy chains linked by disulfide bonds. There are two types of light chains: kappa and lambda, each composed of a constant domain (CL) and a variable domain (VL). There are five types of heavy chains (alpha, gamma, delta, epsilon, and mu), which determines the type of immunoglobulin formed: IgA, IgG, IgD, IgE, and IgM, respectively. In higher vertebrates, there are two types of light chain, designated kappa and lambda, which seem to be functionally identical, and can associate with any of the heavy chains. Members of this group contain standard Ig superfamily V-set AGFCC'C"/DEB domain topology.

                        10        20        30        40        50        60        70        80
                ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
1DBB_L        3 VMTQiPLSLPVNLGDQASISCRSSQSLIHSNgntYLHWYLQKPGQSPKLLMYKVSNRFYGVPDRFSGSGSGTDFTLKISR 82
Cdd:cd04984   1 VLTQ-PSSLSVSPGETVTITCTGSSGNISGN---YVNWYQQKPGSAPRYLIYEDKHRPSGIPDRFSGSKSGNTASLTISG 76

                        90       100
                ....*....|....*....|....*....
1DBB_L       83 VEAEDLGIYFC--SQSSHVppTFGGGTKL 109
Cdd:cd04984  77 AQTEDEADYYCqvWDSNSY--VFGGGTKL 103

Immunoglobulin variable domain (IgV); The members here are composed of the immunoglobulin variable domain (IgV). The IgV family contains the standard Ig superfamily V-set AGFCC'C"/DEB domain topology, and are components of immunoglobulin (Ig) and T cell receptors. The basic structure of Ig molecules is a tetramer of two light chains and two heavy chains linked by disulfide bonds. In Ig, each chain is composed of one variable domain (IgV) and one or more constant domains (IgC); these names reflect the fact that the variability in sequences is higher in the variable domain than in the constant domain. Within the variable domain, there are regions of even more variability called the hypervariable or complementarity-determining regions (CDRs) which are responsible for antigen binding. A predominant feature of most Ig domains is the disulfide bridge connecting 2 beta-sheets with a tryptophan residue packed against the disulfide bond. Ig superfamily (IgSF) domains can be divided into 4 main classes based on their structures and sequences: the Variable (V), Constant 1 (C1), Constant 2 (C2), and Intermediate (I) sets. Typically, the V-set domains have A, B, E and, D strands in one sheet and A', G, F, C, C', and C" strands in the other.

                        10        20        30        40        50        60        70        80
                ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
1DBB_L        4 MTQIPLSLPVNLGDQASISCRSSQSLIHsngnTYLHWYLQKPGQSPKLLMYKVSNR---FYGVPDRFSGSGSG-TDFTLK 79
Cdd:cd00099   1 VTQSPRSLSVQEGESVTLSCEVSSSFSS----TYIYWYRQKPGQGPEFLIYLSSSKgktKGGVPGRFSGSRDGtSSFSLT 76

                        90       100       110
                ....*....|....*....|....*....|....*
1DBB_L       80 ISRVEAEDLGIYFCSQS---SHVPPTFGGGTKLEI 111
Cdd:cd00099  77 ISNLQPEDSGTYYCAVSesgGTDKLTFGSGTRLTV 111

Immunoglobulin Constant-1 (C1)-set domain; The members here are composed of C1-set domains, classical Ig-like domains resembling the antibody constant domain. Members of the IgC1 family are components of immunoglobulin, T-cell receptors, CD1 cell surface glycoproteins, secretory glycoproteins A/C, and major histocompatibility complex (MHC) class I/II molecules. In immunoglobulins, each chain is composed of one variable domain (IgV) and one or more IgC domains. These names reflect the fact that the variability in sequences is higher in the variable domain than in the constant domain. The IgV domain is responsible for antigen binding, while the IgC domain is involved in oligomerization and molecular interactions. The structures in C1-set are smaller than those in the V-set; they have one beta sheet that is formed by strands A, B, E, and D and the other strands by G, F, C, and C'.

                        10        20        30        40        50        60        70        80
                ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
1DBB_L      119 TVSIFPPSSEQLTSGGASVVCFLNNFYPKDINVKWKIDGSERQNGVLNSWTdQDSKDSTYSMSSTLTLTKDEYERHNSYT 198
Cdd:cd00098   1 TVTLLPPSPEEKGGGKVTLVCLVSGFYPKDITVTWLKNGVPLTSGVSTSSP-VEPNDGTYSVTSSLTVPPSDWDEGATYT 79

                        90
                ....*....|....*.
1DBB_L      199 CEATHKTSTSPIVKSF 214
Cdd:cd00098  80 CVVTHESLKSPLSKTW 95

CH3 domain (third constant Ig domain of the heavy chain) in immunoglobulin heavy alpha, gamma, and delta chains, and CH4 domain (fourth constant Ig domain of the heavy chain) in immunoglobulin heavy alpha, epsilon, and mu chains; member of the C1-set of I; The members here are composed of the third and fourth immunoglobulin constant domain (IgC) of alpha, delta, gamma and alpha, epsilon, and mu heavy chains, respectively. This domain is found on the Fc fragment. The basic structure of Ig molecules is a tetramer of two light chains and two heavy chains linked by disulfide bonds. There are two types of light chains: kappa and lambda; each is composed of a constant domain and a variable domain. There are five types of heavy chains: alpha, delta, epsilon, gamma, and mu, all consisting of a variable domain (VH) with three (alpha, delta and gamma) or four (epsilon and mu) constant domains (CH1 to CH4). Ig molecules are modular proteins, in which the variable and constant domains have clear, conserved sequence patterns.

                        10        20        30        40        50        60        70        80
                ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
1DBB_L      118 PTVSIFPPSSEQLTSGG-ASVVCFLNNFYPKDINVKWKIDGSERQNGVLNSWTDQDSKDSTYSMSSTLTLTKDEYERHNS 196
Cdd:cd05768   1 PSVYLLPPPEEELSLNEtVTLTCLVKGFYPEDIFVSWLQNGEPLPSADYKTTAPVPESDGSFFVYSKLNVSTADWNSGDV 80

                        90       100
                ....*....|....*....|.
1DBB_L      197 YTCEATHKTSTSPIV-KSFNR 216
Cdd:cd05768  81 FSCVVGHEALPLQFTqKSIDK 101

Immunoglobulin V-Type;

                           10        20        30        40        50        60        70        80
                   ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
1DBB_L          19 ASISCRSSQSlihSNGNTYLHWYLQKPGQSPKLLMYKVSN--RFY--GVPDRFSGSG--SGTDFTLKISRVEAEDLGIYF 92
Cdd:smart00406   2 VTLSCKFSGS---TFSSYYVSWVRQPPGKGLEWLGYIGSNgsSYYqeSYKGRFTISKdtSKNDVSLTISNLRVEDTGTYY 78

                   ...
1DBB_L          93 CSQ 95
Cdd:smart00406  79 CAV 81

Immunoglobulin V-set domain; This domain is found in antibodies as well as neural protein P0 and CTL4 amongst others.

                          10        20        30        40        50        60        70        80
                  ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
1DBB_L          8 PLSLPVNLGDQASISCrsSQSLIHSNGNTYLHWYLQKPGQSPK--LLMYKVSNRFYGVPDRFSGSG--SGTDFTLKISRV 83
Cdd:pfam07686   3 PREVTVALGGSVTLPC--TYSSSMSEASTSVYWYRQPPGKGPTflIAYYSNGSEEGVKKGRFSGRGdpSNGDGSLTIQNL 80

                          90       100
                  ....*....|....*....|....*...
1DBB_L         84 EAEDLGIYFCSQSSHVPPTFGGGTKLEI 111
Cdd:pfam07686  81 TLSDSGTYTCAVIPSGEGVFGKGTRLTV 108

Immunoglobulin (Ig) variable (V) domain of T-cell receptor (TCR) gamma chain; The members here are composed of the immunoglobulin (Ig) variable (V) domain of the gamma chain of gamma/delta T-cell receptors (TCRs). TCRs mediate antigen recognition by T lymphocytes, and are heterodimers consisting of alpha and beta chains or gamma and delta chains. Each chain contains a variable (V) and a constant (C) region. The majority of T cells contain alpha/beta TCRs, but a small subset contain gamma/delta TCRs. Alpha/beta TCRs recognize antigens as peptide fragments presented by major histocompatibility complex (MHC) molecules. Gamma/delta TCRs recognize intact protein antigens directly without antigen processing and recognize MHC independently of the bound peptide. Gamma/delta T cells can also be stimulated by non-peptide antigens such as small phosphate- or amine-containing compounds. The variable domain of gamma/delta TCRs is responsible for antigen recognition and is located at the N-terminus of the receptor. Members of this group contain the standard Ig superfamily V-set AGFCC'C"/DEB domain topology.

                        10        20        30        40        50        60        70        80
                ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
1DBB_L        4 MTQIPLSLPVNLGDQASISCRSSQSLIHsngNTYLHWYLQKPGQSPKLLMY------------KVSNRFYgvPDRFSGSG 71
Cdd:cd04982   1 LEQPQLSITREESKSVTISCKVSGIDFS---TTYIHWYRQKPGQALERLLYvsstsavrkdsgKTKNKFE--ARKDVGKS 75

                        90       100       110       120
                ....*....|....*....|....*....|....*....|..
1DBB_L       72 SgtdFTLKISRVEAEDLGIYFC----SQSSHVPPTFGGGTKL 109
Cdd:cd04982  76 T---STLTITNLEKEDSATYYCayweSGSGYYIKVFGSGTKL 114

CH2 domain (second constant Ig domain of the heavy chain) in immunoglobulin E (IgE); member of the C1-set of Ig superfamily (IgSF) domains; The members here are composed of the second constant domain of the heavy chain of immunoglobulin E (IgE). The basic structure of immunoglobulin (Ig) molecules is a tetramer of two light chains and two heavy chains linked by disulfide bonds. There are two types of light chains: kappa and lambda; each is composed of a constant domain and a variable domain. There are five types of heavy chains: alpha, delta, epsilon, gamma, and mu, all consisting of a variable domain (VH) with three (alpha, delta, and gamma) or four (epsilon and mu) constant domains (CH1 to CH4). The different classes of antibodies vary in their heavy chains; the IgE class has the epsilon type. This domain (Cepsilon2) of IgE is in place of the flexible hinge region found in IgG.

                        10        20        30        40        50        60        70        80
                ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
1DBB_L      118 PTVSIFPPSS-EQLTSGGASVVCFLNNFYPKDINVKWKIDGSERQNGVLNSWTDQDSkDSTYSMSSTLTLTKDEYERHNS 196
Cdd:cd05847   1 PTVQILHSSCaSTLTSETIQLLCLISGYTPSTIEVEWLVDGQVATLSAASTAPQKEE-GGTFSTTSKLNVTQEDWKSGKT 79

                        90
                ....*....|.
1DBB_L      197 YTCEATHKTST 207
Cdd:cd05847  80 YTCKVTHQGTT 90

Tapasin-R immunoglobulin-like domain; member of the C1-set of Ig superfamily (IgSF) domains; The members here are composed of the immunoglobulin-like domain on Tapasin-R. Tapasin is a V-C1 (variable-constant) immunoglobulin superfamily molecule present in the endoplasmic reticulum (ER), where it links MHC class I molecules to the transporter associated with antigen processing (TAP). Tapasin-R is a tapasin-related protein that contains similar structural motifs to Tapasin, with some marked differences, especially in the V domain, transmembrane and cytoplasmic regions. The majority of Tapasin-R is located within the ER; however, there may be some expression of Tapasin-R at the cell surface. Tapasin-R lacks an obvious ER retention signal.

                        10        20        30        40        50        60        70        80
                ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
1DBB_L      118 PTVSIFPpSSEQLTSGGASVVCFLNNFYPKDINVKWKID---GSERQNGVLNSW--TDQDSKDSTYSMSSTLTLTKDEYE 192
Cdd:cd05771   1 PRVRLSP-KNLVKPDLPQTLSCHIAGYYPLDVDVEWLREepgGSESQVSRDGVSlsSHRQSVDGTYSISSYLTLEPGTEN 79

                        90       100
                ....*....|....*....|.
1DBB_L      193 RHNSYTCEATHKTSTSPIVKS 213
Cdd:cd05771  80 RGATYTCRVTHVSLEEPLSVS 100

Class II major histocompatibility complex (MHC) beta chain immunoglobulin domain of histocompatibility antigen (HLA) DQ and I-A; member of the C1-set of Ig superfamily (IgSF) domains; The members here are composed of the Class II major histocompatibility complex (MHC) beta chain immunoglobulin domain of human histocompatibility antigen (HLA) DQ and mouse I-A. Three genetically distinct isotypes of class II MHC molecules are found in humans (HLA-DR, HLA-DQ, and HLA-DP), and two in mice (I-E and I-A). I-A and I-E have the same basic features insofar as peptide loading and presentation, they differ in that each interacts with distinctly different sets of peptides, and in the incidence of deletion of their genes. A structural understanding of the similarities and differences between I-A and I-E may help with understanding their roles in peptide presentation and T cell activation. Mouse I-Ag7 has a genetic susceptibility to autoimmune diabetes due to its small, uncharged amino acid residue at position 57 of their beta chain which results in the absence of a salt bridge between beta 57 and Arg alpha 76, which is adjacent to the P9 pocket of the peptide-binding groove. Human HLA-DR, -DQ, and -DP are about 70% similar to each other. HLA-DQ (DQ) is a cell surface receptor protein found on antigen presenting cells. It is an alphabeta heterodimer of type MHC class II. The alpha and beta chains are encoded by two loci, HLA-DQA1 and HLA-DQB1, that are adjacent to each other on chromosome band 6p21.3. A person often produces two alpha-chain and two beta chain variants and thus 4 isoforms of DQ. HLA-DQ is involved in the autoimmune diseases celiac disease and diabetes mellitus type. DQ is one of several antigens involved in rejection of organ transplants. DQ2 is encoded by the HLA-DQB1*02 allele group. DQ6 is encoded by the HLA-DQB1*06 allele group. DQ2 beta-chains combine with alpha-chains, encoded by genetically linked HLA-DQA1 alleles, to form the cis-haplotype isoforms. These isoforms, nicknamed DQ2.2 and DQ2.5, are also encoded by the DQA1*0201 and DQA1*0501 genes, respectively. DQ6 beta-chains combine with alpha-chains, encoded by genetically linked HLA-DQA1 alleles, to form the cis-haplotype isoforms. For DQ6, however, cis-isoform pairing only occurs with DQ1 alpha-chains. There are many haplotypes of DQ6. Susceptibility to Leptospirosis infection was found associated with undifferentiated DQ6. DQ8 is determined by the antibody recognition of beta8 and this generally detects the gene product of DQB1*0302. DQ8 is commonly linked to autoimmune disease in the human population. DQ8 is the second most predominant isoform linked to celiac disease and the DQ most linked to Type 1 diabetes. DQ8 increases the risk for rheumatoid arthritis and is linked to the primary risk locus for RA, HLA-DR4. DR4 also plays an important role in Type 1 diabetes. DQ8 is a split antigen of the DQ3 broad antigen. MHC class II molecules play a key role in the initiation of the antigen-specific immune response. They are expressed constitutively on the cell surface of professional antigen-presenting cells (APCs), including B-lymphocytes, monocytes, and macrophages in both humans and mice, and induced in nonprofessional APCs, such as keratinocyctes; they are expressed on the surface of activated human T cells and on T cells from other species. MHC II molecules present antigenic peptides to CD4(+) T-lymphocytes; these peptides derive mostly from proteolytic processing via the endocytic pathway, of antigens internalized by the APC, and bind to the MHC class II molecules in the endosome before they are transported to the cell surface. MHC class II molecules are heterodimers, comprised of two similarly-sized membrane-spanning chains, alpha and beta. Each chain had two globular domains (N- and C-terminal), and a membrane-anchoring transmembrane segment. The two chains form a compact four-domain structure. The peptide-binding site is a cleft in the structure.

                        10        20        30        40        50        60        70        80
                ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
1DBB_L      116 AAPTVSIFPPSSEQLTSGGAsVVCFLNNFYPKDINVKWKIDGSERQNGVLNSWTDQDSkDSTYSMSSTLTLTKdeyERHN 195
Cdd:cd21001   2 VEPTVTISPSRTEALNHHNL-LVCSVTDFYPGQIKVRWFRNDQEETAGVVSTPLIRNG-DWTFQILVMLEMTP---QRGD 76

                        90
                ....*....|....*
1DBB_L      196 SYTCEATHKTSTSPI 210
Cdd:cd21001  77 VYTCHVEHPSLQSPI 91

Class II major histocompatibility complex (MHC) beta chain immunoglobulin domain of histocompatibility antigen (HLA) I-E; member of the C1-set of Ig superfamily (IgSF) domains; The members here are composed of the Class II major histocompatibility complex (MHC) beta chain immunoglobulin domain of histocompatibility antigen (HLA) I-E. Three genetically distinct isotypes of class II MHC molecules are found in humans (HLA-DR, HLA-DQ, and HLA-DP), and two in mice (I-E and I-A). I-A and I-E molecules have the same basic features insofar as peptide loading and presentation, although each interacts with distinctly different sets of peptides. They also differ in that there is a relatively high incidence of deletion of the I-E gene in both inbred strains of mice as well as wild mice and the lack of the reverse situation i.e. the deletion of I-A genes. A detailed structural understanding of the similarities and differences between I-A and the paralogous I-E could help illuminate the respective roles these molecules play in peptide presentation and T cell activation. Mouse I-Ag7 has a genetic susceptibility to autoimmune diabetes due to its small, uncharged amino acid residue at position 57 of their beta chain which results in the absence of a salt bridge between beta 57 and Arg alpha 76, which is adjacent to the P9 pocket of the peptide-binding groove. MHC class II molecules play a key role in the initiation of the antigen-specific immune reponse. These molecules have been shown to be expressed constitutively on the cell surface of professional antigen-presenting cells (APCs), including B-lymphocytes, monocytes, and macrophages in both humans and mice. The expression of these molecules has been shown to be induced in nonprofessional APCs such as keratinocyctes, and they are expressed on the surface of activated human T cells and on T cells from other species. The MHC II molecules present antigenic peptides to CD4(+) T-lymphocytes. These peptides derive mostly from proteolytic processing via the endocytic pathway, of antigens internalized by the APC. These peptides bind to the MHC class II molecules in the endosome before they are transported to the cell surface. MHC class II molecules are heterodimers, comprised of two similarly-sized membrane-spanning chains, alpha and beta. Each chain had two globular domains (N- and C-terminal), and a membrane-anchoring transmembrane segment. The two chains form a compact four-domain structure. The peptide-binding site is a cleft in the structure.

                        10        20        30        40        50        60        70        80
                ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
1DBB_L      118 PTVSIFPPSSEQLTSGGAsVVCFLNNFYPKDINVKWKIDGSERQNGVLNSWTDQDSkDSTYsmsSTLTLTKDEYERHNSY 197
Cdd:cd20998   7 PTVTVYPTKTQPLEHHNL-LVCSVSDFYPGNIEVRWFRNGKEEKTGIVSTGLVRNG-DWTF---QTLVMLETVPQSGEVY 81

                        90
                ....*....|...
1DBB_L      198 TCEATHKTSTSPI 210
Cdd:cd20998  82 TCQVEHPSLTDPV 94

Class II major histocompatibility complex (MHC) beta chain immunoglobulin domain; member of the C1-set of Ig superfamily (IgSF) domains; The members here are composed of the immunoglobulin (Ig) domain of major histocompatibility complex (MHC) class II beta chain. MHC class II molecules play a key role in the initiation of the antigen-specific immune reponse. These molecules have been shown to be expressed constitutively on the cell surface of professional antigen-presenting cells (APCs), including B-lymphocytes, monocytes, and macrophages in both humans and mice. The expression of these molecules has been shown to be induced in nonprofessional APCs such as keratinocyctes and they are also expressed on the surface of activated human T cells and on T cells from other species. The MHC II molecules present antigenic peptides to CD4(+) T-lymphocytes. These peptides derive mostly from proteolytic processing via the endocytic pathway of antigens internalized by the APC. These peptides bind to the MHC class II molecules in the endosome before they are transported to the cell surface. MHC class II molecules are heterodimers, comprised of two similarly-sized membrane-spanning chains, alpha and beta. Each chain has two globular domains (N- and C-terminal) and a membrane-anchoring transmembrane segment. The two chains form a compact four-domain structure. The peptide-binding site is a cleft in the structure.

                        10        20        30        40        50        60        70        80
                ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
1DBB_L      118 PTVSIFPPSSEQLTSGgASVVCFLNNFYPKDINVKWKIDGSERQNGVlNSWTDQDSKDSTYSMSSTLTLTKdeyERHNSY 197
Cdd:cd05766   4 PSVKVSPTKTGPLEHP-NLLVCSVTGFYPAEIEVKWFRNGQEETAGV-VSTELIPNGDWTFQILVMLETTP---RRGDVY 78

                        90
                ....*....|...
1DBB_L      198 TCEATHKTSTSPI 210
Cdd:cd05766  79 TCQVEHSSLQSPL 91

Signal-regulatory protein (SIRP) immunoglobulin-like domain 2; member of the C1-set of Ig superfamily (IgSF) domains; The members here are composed of the immunoglobulin (Ig)-like domain in Signal-Regulatory Protein (SIRP), domain 2 (C1 repeat 1). The SIRPs belong to the "paired receptors" class of membrane proteins that comprise several genes coding for proteins with similar extracellular regions, but very different transmembrane/cytoplasmic regions with different (activating or inhibitory) signaling potentials. They are commonly on NK cells, but are also on many myeloid cells. Their extracellular region contains three Immunoglobulin superfamily domains, a single V-set and two C1-set IgSF domains. Their cytoplasmic tails contain either ITIMs or transmembrane regions that have positively charged residues that allow an association with adaptor proteins, such as DAP12/KARAP, containing ITAMs. There are 3 distinct SIRP members: alpha, beta, and gamma. SIRP alpha (also known as CD172a or SRC homology 2 domain-containing protein tyrosine phosphatase substrate 1/Shps-1) is a membrane receptor that interacts with a ligand CD47 expressed on many cells and gives an inhibitory signal through immunoreceptor tyrosine-based inhibition motifs in the cytoplasmic region that interact with phosphatases SHP-1 and SHP-2. SIRP beta has a short cytoplasmic region and associates with a transmembrane adapter protein DAP12 containing immunoreceptor tyrosine-based activation motifs to give an activating signal. SIRP gamma contains a very short cytoplasmic region lacking obvious signaling motifs, but also binds CD47, but with much less affinity.

                        10        20        30        40        50        60        70        80
                ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
1DBB_L      134 GASVVCFLNNFYPKDINVKWKIDGSErqngVLNSWTD-QDSKDS-TYSMSSTLTLTKDEYERHNSYTCEATHKTSTSPIV 211
Cdd:cd05772  19 TVSFTCKSHGFSPRDITLKWFKNGNE----LSALQTTvFPEGDSvSYSVSSTVQVVLTKDDVHSQLTCEVAHVTLQAPLR 94

                ....
1DBB_L      212 KSFN 215
Cdd:cd05772  95 GTAN 98

Class II major histocompatibility complex (MHC) alpha chain immunoglobulin domain; member of the C1-set of Ig superfamily (IgSF) domains; The members here are composed of the immunoglobulin (Ig) domain of the major histocompatibility complex (MHC) class II alpha chain. MHC class II molecules play a key role in the initiation of the antigen-specific immune reponse. These molecules have been shown to be expressed constitutively on the cell surface of professional antigen-presenting cells (APCs), including B-lymphocytes, monocytes, and macrophages in both humans and mice. The expression of these molecules has been shown to be induced in nonprofessional APCs such as keratinocyctes, and they are also expressed on the surface of activated human T cells and on T cells from other species. The MHC II molecules present antigenic peptides to CD4(+) T-lymphocytes. These peptides derive mostly from proteolytic processing via the endocytic pathway, of antigens internalized by the APC. These peptides bind to the MHC class II molecules in the endosome before they are transported to the cell surface. MHC class II molecules are heterodimers, comprised of two similarly-sized membrane-spanning chains, alpha and beta. Each chain had two globular domains (N- and C-terminal), and a membrane-anchoring transmembrane segment. The two chains form a compact four-domain structure. The peptide-binding site is a cleft in the structure.

                        10        20        30        40        50        60        70        80
                ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
1DBB_L      117 APTVSIFPpsSEQLTSGGASV-VCFLNNFYPKDINVKWKIDGSERQNGVlnSWTD-QDSKDSTYSMSSTLTLTKdeyERH 194
Cdd:cd05767   2 PPEVTVFP--KSPVELGEPNTlICFVDNFFPPVINVTWLRNGQPVTDGV--SETVfLPREDHSFRKFSYLPFTP---SEG 74

                        90
                ....*....|....*...
1DBB_L      195 NSYTCEATHKTSTSPIVK 212
Cdd:cd05767  75 DIYDCRVEHWGLEEPLLK 92

Immunoglobulin (Ig) variable (V) domain of Cluster of Differentiation (CD) 8 beta chain; The members here are composed of the immunoglobulin (Ig)-like domain in Cluster of Differentiation (CD) 8 beta. The CD8 glycoprotein plays an essential role in the control of T-cell selection, maturation, and the T-cell receptor (TCR)-mediated response to peptide antigen. CD8 is comprised of alpha and beta subunits and is expressed as either an alpha/alpha or alpha/beta dimer. Both dimeric isoforms can serve as a coreceptor for T cell activation and differentiation, however they have distinct physiological roles, different cellular distributions, unique binding partners, etc. Each CD8 subunit is comprised of an extracellular domain containing a V-type Ig-like domain, a single pass transmembrane portion, and a short intracellular domain. Members of this group contain standard Ig superfamily V-set AGFCC'C"/DEB domain topology.

                        10        20        30        40        50        60        70        80
                ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
1DBB_L        6 QIPLSLPVNLGDQASISCRSSQSLihsnGNTYLHWYLQKPGQSP----KLLMYKVSNR--FYG---VPDRFS--GSGSGT 74
Cdd:cd07700   3 QTPGSLLVQTNQTVKMSCEAKTSP----KNTRIYWLRQRQAPSKdshfEFLASWDPSKgiVYGegvDQEKLIilSDSDSS 78

                        90       100       110
                ....*....|....*....|....*....|....*....
1DBB_L       75 DFTLKISRVEAEDLGIYFCS--QSSHVppTFGGGTKLEI 111
Cdd:cd07700  79 RYILSLMSVKPEDSGTYFCMtvGSPEL--IFGTGTKLSV 115

Class II major histocompatibility complex (MHC) beta chain immunoglobulin domain of histocompatibility antigen (HLA) DP; member of the C1-set of Ig superfamily (IgSF) domains; The members here are composed of the Class II major histocompatibility complex (MHC) beta chain immunoglobulin domain of histocompatibility antigen (HLA) DP. HLA class II histocompatibility antigen, DP(W2) beta chain is a protein that in humans is encoded by the HLA-DPB1 gene. It plays a central role in the immune system by presenting peptides derived from extracellular proteins. MHC class II molecules are encoded by three different loci, HLA-DR, -DQ, and -DP, which are about 70% similar to each other. HLA-DP is an alphabeta heterodimer cell-surface receptor. Each DP subunit (alpha-subunit, beta-subunit) is composed of a alpha-helical N-terminal domain, an IgG-like beta sheet, a membrane spanning domain, and a cytoplasmic domain. The alpha-helical domain forms the sides of the peptide binding groove. The beta sheet regions form the base of the binding groove and the bulk of the molecule as well as the inter-subunit (non-covalent) binding region. Individuals carrying the MHCII allele, HLA-DP2, are at risk for chronic beryllium disease (CBD), a debilitating inflammatory lung condition caused by the reaction of CD4 T cells to inhaled beryllium. MHC class II molecules play a key role in the initiation of the antigen-specific immune reponse. These molecules have been shown to be expressed constitutively on the cell surface of professional antigen-presenting cells (APCs), including B-lymphocytes, monocytes, and macrophages in both humans and mice. The expression of these molecules has been shown to be induced in nonprofessional APCs such as keratinocyctes, and they are expressed on the surface of activated human T cells and on T cells from other species. The MHC II molecules present antigenic peptides to CD4(+) T-lymphocytes. These peptides derive mostly from proteolytic processing via the endocytic pathway, of antigens internalized by the APC. These peptides bind to the MHC class II molecules in the endosome before they are transported to the cell surface. MHC class II molecules are heterodimers, comprised of two similarly-sized membrane-spanning chains, alpha and beta. Each chain had two globular domains (N- and C-terminal), and a membrane-anchoring transmembrane segment. The two chains form a compact four-domain structure. The peptide-binding site is a cleft in the structure.

                        10        20        30        40        50        60        70
                ....*....|....*....|....*....|....*....|....*....|....*....|....*....|...
1DBB_L      138 VCFLNNFYPKDINVKWKIDGSERQNGVLNSWTDQDSkDSTYSMSSTLTLTKdeyERHNSYTCEATHKTSTSPI 210
Cdd:cd21003  23 VCHVTDFYPGNIQVRWFLNGQEETAGVVSTNLIHNG-DWTFQILVMLEMTP---QQGDVYTCQVEHPSLDSPV 91

Immunoglobulin (Ig) variable (V) domain of T-cell receptor (TCR) delta chain; The members here are composed of the immunoglobulin (Ig) variable (V) domain of the delta chain of gamma/delta T-cell receptors (TCRs). TCRs mediate antigen recognition by T lymphocytes, and are heterodimers consisting of alpha and beta chains or gamma and delta chains. Each chain contains a variable (V) and a constant (C) region. The majority of T cells contain alpha/beta TCRs, but a small subset contain gamma/delta TCRs. Alpha/beta TCRs recognize antigen as peptide fragments presented by major histocompatibility complex (MHC) molecules. Gamma/delta TCRs recognize intact protein antigens; they recognize protein antigens directly and without antigen processing, and MHC independently of the bound peptide. Gamma/delta T cells can also be stimulated by non-peptide antigens such as small phosphate- or amine-containing compounds. The variable domain of gamma/delta TCRs is responsible for antigen recognition and is located at the N-terminus of the receptor. Members of this group contain standard Ig superfamily V-set AGFCC'C"/DEB domain topology.

                        10        20        30        40        50        60        70        80
                ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
1DBB_L        5 TQIPLSLPVNLGDQASISCRSSQSlihsNGNTYLHWYLQKPGQSPKLLMYKVSNRFYGVPDRFSGSGSGTD--FTLKISR 82
Cdd:cd07706   3 TQAQPDVSVQVGEEVTLNCRYETS----WTNYYLFWYKQLPSGEMTFLIRQDSSEQNAKSGRYSVNFQKAQksISLTISA 78

                        90       100       110
                ....*....|....*....|....*....|..
1DBB_L       83 VEAEDLGIYFCSQSSHVPP---TFGGGTKLEI 111
Cdd:cd07706  79 LQLEDSAKYFCALSLPYDTdklIFGKGTRLTV 110

Class Ib major histocompatibility complex (MHC) immunoglobulin domain of human leukocyte antigen (HLA) F; member of the C1-set of Ig superfamily (IgSF) domains; The members here are composed of the Class Ib major histocompatibility complex (MHC) immunoglobulin domain of human leukocyte antigen alpha chain F (HLA-F). HLA-F, encoded by the HLA-F gene in humans, belongs to the non-classical HLA class I heavy chain paralogs. This class I molecule mainly exists as a heterodimer associated with the invariant light chain beta-2-microglobulin. HLA-F molecules can interact with both activating and inhibitory receptors on immune cells, such as NK cells, and can present a diverse panel of peptides. Class I MHC proteins bind antigenic peptide fragments and present them to CD8+ T lymphocytes. Class I molecules consist of a transmembrane alpha chain and a small chain called the beta-2-microglobulin. The alpha chain contains three extracellular domains, two of which fold together to form the peptide-binding cleft (alpha1 and alpha2), and one which has an Ig fold (alpha3). Peptide binding to class I molecules occurs in the endoplasmic reticulum (ER) and involves both chaperones and dedicated factors to assist in peptide loading. Class I MHC molecules are expressed on most nucleated cells.

                        10        20        30        40        50        60        70
                ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*..
1DBB_L      135 ASVVCFLNNFYPKDINVKWKIDGSERQNGVLNSWTdQDSKDSTYSMSSTLTLTKDEYERhnsYTCEATHKTSTSPIV 211
Cdd:cd21023  20 ATLRCWALGFYPAEITLTWQRDGEEQTQDTELVET-RPAGDGTFQKWAAVVVPPGEEQR---YTCHVQHEGLPQPLI 92

Immunoglobulin (Ig)-like domain of platelet-derived growth factor (PDGF) receptors (R), alpha and beta; member of the I-set of IgSF domains; The members here are composed of the immunoglobulin (Ig)-like domain of platelet-derived growth factor (PDGF) receptors (R), alpha (also known as cluster of differentiation (CD) 140a), and beta (also known as CD140b). PDGF is a potent mitogen for connective tissue cells. PDGF-stimulated processes are mediated by three different PDGFs (PDGF-A,PDGF-B, and PDGF-C). PDGFRalpha binds to all three PDGFs, whereas the PDGFRbeta binds only to PDGF-B. PDGFRs alpha and beta have similar organization: an extracellular component with five Ig-like domains, a transmembrane segment, and a cytoplasmic portion having protein tyrosine kinase activity. In mice, PDGFRalpha and PDGFRbeta are essential for normal development.

                        10        20        30        40        50        60
                ....*....|....*....|....*....|....*....|....*....|....*....|....*..
1DBB_L      149 INVKWKIDGSERQNGVLNSwtdQDSKDSTYSMSSTLTLTKDEYERHNSYTCEATHKTSTSPIVKSFN 215
Cdd:cd05861  32 VDLEWTYPGKESGRGIEPV---EEFKVPPYHLVYTLTIPSATLEDSGTYECAVTEATNDHQDEKKIN 95

Class II major histocompatibility complex (MHC) alpha chain immunoglobulin domain of histocompatibility antigen (HLA) I-A; member of the C1-set of Ig superfamily (IgSF) domains; The members here are composed of the immunoglobulin (Ig) domain of major histocompatibility complex (MHC) class II alpha chain of histocompatibility antigen (HLA) I-A. Three genetically distinct isotypes of class II MHC molecules are found in humans (HLA-DR, HLA-DQ, and HLA-DP), and two in mice (I-E and I-A). I-A and I-E molecules have the same basic features insofar as peptide loading and presentation, although each interacts with distinctly different sets of peptides. They also differ in that there is a relatively high incidence of deletion of the I-E a gene in both inbred strains of mice as well as wild mice and the lack of the reverse situation i.e. the deletion of I-A genes. A detailed structural understanding of the similarities and differences between I-A and the paralogous I-E could help illuminate the respective roles these molecules play in peptide presentation and T cell activation. Mouse I-Ag7 has a genetic susceptibility to autoimmune diabetes due to its small, uncharged amino acid residue at position 57 of their beta chain which results in the absence of a salt bridge between beta 57 and Arg alpha 76, which is adjacent to the P9 pocket of the peptide-binding groove. MHC class II molecules play a key role in the initiation of the antigen-specific immune reponse. These molecules have been shown to be expressed constitutively on the cell surface of professional antigen-presenting cells (APCs), including B-lymphocytes, monocytes, and macrophages in both humans and mice. The expression of these molecules has been shown to be induced in nonprofessional APCs such as keratinocyctes, and they are expressed on the surface of activated human T cells and on T cells from other species. The MHC II molecules present antigenic peptides to CD4(+) T-lymphocytes. These peptides derive mostly from proteolytic processing via the endocytic pathway, of antigens internalized by the APC. These peptides bind to the MHC class II molecules in the endosome before they are transported to the cell surface. MHC class II molecules are heterodimers, comprised of two similarly-sized membrane-spanning chains, alpha and beta. Each chain had two globular domains (N- and C-terminal), and a membrane-anchoring transmembrane segment. The two chains form a compact four-domain structure. The peptide-binding site is a cleft in the structure.

                        10        20        30        40        50        60        70        80
                ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
1DBB_L      117 APTVSIFPpSSEQLTSGGASVVCFLNNFYPKDINVKWKIDGSERQNGVLNSwTDQDSKDSTYSMSSTLTLTKDEyerHNS 196
Cdd:cd21006   2 APQATVFP-KSPVLLGQPNTLICFVDNIFPPVINITWLRNSKSVTDGVYET-SFLVNRDHSFHKLSYLTFIPSD---DDI 76

                        90
                ....*....|....*....
1DBB_L      197 YTCEATHKTSTSPIVKSFN 215
Cdd:cd21006  77 YDCKVEHWGLEEPVLKHWE 95

Class Ib major histocompatibility complex (MHC) immunoglobulin domain of human leukocyte antigen H; member of the C1-set of Ig superfamily (IgSF) domains; The members here are composed of the Class Ib major histocompatibility complex (MHC) immunoglobulin domain of human leukocyte antigen H (HLA-H). HLA-H (also known as hereditary hemochromatosis protein; HFE) is a major histocompatibility complex (MHC) class I-like protein that is mutated in Hereditary Hemochromatosis. HFE is a protein of 343 amino acids that includes a signal peptide, an extracellular transferrin receptor-binding region (a1 and a2), an immunoglobulin-like domain (a3), a transmembrane region, and a short cytoplasmic tail. HFE binds beta-2-microglobulin to form a heterodimer expressed at the cell surface. It binds transferrin receptor (TFRC) in its extracellular alpha1-alpha2 domain. HFE plays an important part in the regulation of hepcidin expression in response to iron overload and the liver is important in the pathophysiology of HFE-associated hemochromatosis. Nine HFE splicing variants have been reported with transcripts lacking exon 2 or exon 3, or exons 2-3, 2-4, or 2-5. Diverse mutations involving HFE introns and exons discovered in persons with hemochromatosis or their family members cause or probably cause high iron phenotypes. Class I MHC proteins bind antigenic peptide fragments and present them to CD8+ T lymphocytes. Class I molecules consist of a transmembrane alpha chain and a small chain called the beta-2-microglobulin. The alpha chain contains three extracellular domains, two of which fold together to form the peptide-binding cleft (alpha1 and alpha2), and one which has an Ig fold (alpha3). Peptide binding to class I molecules occurs in the endoplasmic reticulum (ER) and involves both chaperones and dedicated factors to assist in peptide loading. Class I MHC molecules are expressed on most nucleated cells.

                        10        20        30        40        50        60        70        80
                ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
1DBB_L      124 PP---SSEQLTSGGASVVCFLNNFYPKDINVKWKIDGSERQNGVLNSWTDQDSKDSTYSMSSTLTLTKDEYERhnsYTCE 200
Cdd:cd21021   4 PPlvkVTHHVTSSVTTLRCRALNYYPQNITMKWLKDKQPMDAKEFEPKDVLPNGDGTYQGWITLAVPPGEEQR---YTCQ 80

                        90
                ....*....|.
1DBB_L      201 ATHKTSTSPIV 211
Cdd:cd21021  81 VEHPGLDQPLI 91

Immunoglobulin (Ig)-like domain of Programmed Cell Death 1 (PD1); The members here are composed of the immunoglobulin (Ig)-like domain of Programmed Cell Death 1 (PD1; also known as CD279/cluster of differentiation 279). PD1 is a cell surface receptor that is expressed on T cells and pro-B cells. The protein's structure includes an extracellular IgV domain followed by a transmembrane region and an intracellular tail. Activation of CD4+ T cells, CD8+ T cells, NKT cells, B cells, and monocytes induces PD-1 expression, immediately after which it binds two distinct ligands, PD-L1 (also known as B7-H1 or CD274/cluster of differentiation 274) and PD-L2, also known as B7-DC. PD-1 plays an important role in down regulating the immune system by preventing the activation of T-cells, reducing autoimmunity and promoting self-tolerance. The inhibitory effect of PD-1 is accomplished by promoting apoptosis in antigen specific T-cells in lymph nodes while simultaneously reducing apoptosis in regulatory T cells. A class of drugs that target PD-1, known as the PD-1 inhibitors, activate the immune system to attack tumors and treat cancer. Comparisons between the mouse PD-1 (mPD-1) and human PD-1 (hPD-1) reveals that unlike the mPD-1 which has a conventional IgSF V-set domain, hPD-1 lacks a C" strand, and instead the C' and D strands are connected by a long and flexible loop. In addition, the BC loop is not stabilized by disulfide bonding to the F strand of the ligand binding beta sheet. These differences result in different binding affinities of human and mouse PD-1 for their ligands.

                        10        20        30        40        50        60        70        80
                ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
1DBB_L        8 PLSLPVNLGDQASISCrssqSLIHSNGNTYLHWYLQKPG-QSPKLLMYKVSNRFYGVPDRFSGSG--SGTDFTLKISRVE 84
Cdd:cd16088   6 PALLVVTEGANATFTC----SFSNTSESFVLNWYRLSPSnQTDKLAAFPEDRSQPGQDWRFRVTQlpNGRDFHMSVVRAR 81

                        90
                ....*....|....*...
1DBB_L       85 AEDLGIYFCSQSSHVPPT 102
Cdd:cd16088  82 RNDSGTYLCGAISLAPKA 99

Gammadelta T-cell antigen receptor, variable (V) domain; The members here are composed of the immunoglobulin (Ig) variable (V) domain of the gamma/delta T-cell receptors (TCRs). TCRs mediate antigen recognition by T lymphocytes, and are heterodimers consisting of alpha and beta chains or gamma and delta chains. Each chain contains a variable (V) and a constant (C) region. The majority of T cells contain alpha/beta TCRs, but a small subset contain gamma/delta TCRs. Alpha/beta TCRs recognize antigen as peptide fragments presented by major histocompatibility complex (MHC) molecules. Gamma/delta TCRs recognize intact protein antigens; they recognize protein antigens directly and without antigen processing, and MHC independently of the bound peptide. Gamma/delta T cells can also be stimulated by non-peptide antigens such as small phosphate- or amine-containing compounds. The variable domain of gamma/delta TCRs is responsible for antigen recognition and is located at the N-terminus of the receptor. Members of this group contain standard Ig superfamily V-set AGFCC'C"/DEB domain topology.

                        10        20        30        40        50        60        70        80
                ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
1DBB_L       10 SLPVNLGDQASISCRSSQSLIhsnGNTYLHWYLQKPGQSPKLLMYKVSNRFYGVPDRFSGS--GSGTDFTLKISRVEAED 87
Cdd:cd20988   7 TVTVSVGKPVTLKCSMKGEAI---SNYYINWYRKTQGNTMTFIYREGGIYGPGFKDNFRGDidSSNNLAVLKILEASERD 83

                        90       100       110
                ....*....|....*....|....*....|.
1DBB_L       88 LGIYFCSQSS------HVPPTFGGGTKLEIK 112
Cdd:cd20988  84 EGSYYCASDTpgggreYDPLIFGKGTYLTVE 114

Class II major histocompatibility complex (MHC) alpha chain immunoglobulin domain of histocompatibility antigen (HLA) DQ and related proteins; member of the C1-set of Ig superfamily (IgSF) domains; The members here are composed of the immunoglobulin (Ig) domain of major histocompatibility complex (MHC) class II alpha chain of histocompatibility antigen (HLA) DQ. MHC class II molecules are encoded by three different loci, HLA-DR, -DQ, and -DP, which are about 70% similar to each other. HLA-DQ (DQ) is a cell surface receptor protein found on antigen presenting cells. It is an alphabeta heterodimer of type MHC class II. The alpha and beta chains are encoded by two loci, HLA-DQA1 and HLA-DQB1, that are adjacent to each other on chromosome band 6p21.3. A person often produces two alpha-chain and two beta chain variants and thus 4 isoforms of DQ. Two autoimmune diseases in which HLA-DQ is involved are celiac disease and diabetes mellitus type 1. DQ is one of several antigens involved in rejection of organ transplants. DQ8 is a split antigen of the DQ3 broad antigen. MHC class II molecules play a key role in the initiation of the antigen-specific immune reponse. These molecules have been shown to be expressed constitutively on the cell surface of professional antigen-presenting cells (APCs), including B-lymphocytes, monocytes, and macrophages in both humans and mice. The expression of these molecules has been shown to be induced in nonprofessional APCs such as keratinocyctes, and they are expressed on the surface of activated human T cells and on T cells from other species. The MHC II molecules present antigenic peptides to CD4(+) T-lymphocytes. These peptides derive mostly from proteolytic processing via the endocytic pathway, of antigens internalized by the APC. These peptides bind to the MHC class II molecules in the endosome before they are transported to the cell surface. MHC class II molecules are heterodimers, comprised of two similarly-sized membrane-spanning chains, alpha and beta. Each chain had two globular domains (N- and C-terminal), and a membrane-anchoring transmembrane segment. The two chains form a compact four-domain structure. The peptide-binding site is a cleft in the structure.

                        10        20        30        40        50        60        70        80
                ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
1DBB_L      118 PTVSIFPPSSEQLTSGGaSVVCFLNNFYPKDINVKWKIDGSERQNGVlnSWTDQDSK-DSTYSMSSTLTLTKDEYErhnS 196
Cdd:cd21008   3 PEVTVFPKSPVTLGQPN-TLICLVDNIFPPVINITWLSNGHSVTEGV--SETSFLSKsDHSFLKISYLTFLPSADD---I 76

                        90
                ....*....|....*.
1DBB_L      197 YTCEATHKTSTSPIVK 212
Cdd:cd21008  77 YDCKVEHWGLDKPLLK 92