Ig2_CD4; second immunoglobulin (Ig) domain of CD4. CD4 and CD8 are the two primary co-receptor proteins found on the surface of T cells, and the presence of either CD4 or CD8 determines the function of the T cell. CD4 is found on helper T cells, where it is required for the binding of MHC (major histocompatibility complex) class II molecules, while CD8 is found on cytotoxic T cells, where it is required for the binding of MHC class I molecules. CD4 contains four immunoglobulin domains, with the first three included in this hierarchy. The fourth domain has a general Ig architecture, but has slight topological changes in the arrangement of beta strands relative to the other structures in this family and is not specifically included in the hierarchy.