Mice deficient for paired immunoglobulin (Ig)-like receptor B (PIR-B) show defective regulation of receptor-mediated activation in antigen-presenting cells. Older PIR-B(-/-) mice had an increased number of peritoneal B1 cells. Splenic PIR-B(-/-) B2 cells were constitutively activated and proliferated much more than those from wild-type mice upon B cell receptor ligation. T helper type 2 (T(H)2)-prone humoral responses were augmented in PIR-B(-/-) mice upon immunization with T-dependent antigens, including increased interleukin 4 and decreased interferon-gamma responses, as well as enhanced IgG1 and IgE production. Impaired maturation of dendritic cells (DCs), possibly due to perturbed intracellular signaling, was responsible for the skewed responses. Thus, PIR-B is critical for B cell suppression, DC maturation and for balancing T(H)1 and T(H)2 immune responses.