CD5 is a type I transmembrane glycoprotein with a molecular weight of 67 kDa. The extracellular domain consists of 3 scavenger receptor cysteine-rich (SRCR) domains of nearly 100 amino acids each. The CD5 is expressed at the surface of mature T lymphocytes, by most of thymocytes and by a subpopulation of B lymphocytes (B-1a) expanded in neonatal life, several autoimmune disorders and some B-cell proliferative disorders (B-CLL). It is not found on granulocytes, monocytes and platelets. The CD5 antigen is the ligand for the B-lymphocytes cell-surface protein CD72. The CD5 / CD72 interactions are involved in the regulation of T and B lymphocyte activation and proliferation. B lymphocytes can be divided into B-1 lymphocytes and B-2 (or conventional B) lymphocytes based on different localization, functional characteristics and gene expression. The B-1 lymphocyte subset is divided into B-1a and B-1b subpopulations, which do or do not express the CD5 antigen respectively. The B-1a CD5+ B lymphocyte subset expresses immunoglobulin with inherent low affinities for self-antigens.