CD5 appears to be a relatively late marker during B cell differentiation. CD5 expression is thought to be absent on surface Ig negative B-lineage cells but appears on IgM+ cells in both fetal liver and bone marrow. It co-precipitates with the T cell receptor and, in particular with Lck. A role for CD5 in signal transduction is postulated based on stimulatory effects of CD5 monoclonal antibodies. CD5 antigen is phosphorylated on tyrosine residues on T cell activation. There is evidence that CD5 plays a role in thymocyte selection, as well as a role in cell-cell recognition. Recently CD5 on B cells has been shown to be an endogenous ligand selective for B-cell surface IgFR (framework region) sequences. Interaction of surface Ig with CD5, other endogenous antigens or (in mucosal sites) exogenous superantigens can provide B cells with continual stimulation and might prevent their elimination from the immune system. In addition, B cell superantigens, e.g. Staphylococcus aureus Cowan strain 1, may contribute to the pathogenesis of autoimmune diseases and malignancies.Synonyms: CD5, LEU1, Lymphocyte antigen T1/Leu-1, T-cell surface glycoprotein CD5