CD5 is a type I transmembrane protein belonging to the scavenger receptor cysteine-enriched (SRCR) family characterized by the presence of at least one SRCR domain containing 90-110 amino acids CD5 is expressed in all mature T cells B-1A subsets of mature B cells and certain leukemia B cells It is elevated in regulatory T and B cells (Tregs Bregs) CD5 expression was also elevated in incompetent T and B cells Elevated levels of CD5 have also been found in many autoimmune diseases CD5 binds to T cell receptors (TCR) and negatively regulates T cell activation and differentiation The expression of CD5 in tumor-infiltrating T lymphocytes was negatively correlated with their antitumor activity Recently it has been reported that CD5 binds directly to IL6 and mediates downstream signal transduction CD5 B cells promote tumor growth in animal models Agents targeting CD5 are being actively explored as therapeutic interventions for cancer and other conditions