CD5 (protein)

CD5 is a cluster of differentiation expressed on the surface of T cells and in a subset of murine B cells known as B-1a. The expression of this receptor in human B cells has been a controversial topic and to date there is no consensus regarding the role of this receptor as a marker of human B cells. B-1 cells have limited diversity of their B-cell receptor due to their lack of the enzyme terminal deoxynucleotidyl transferase (TdT) and are potentially self-reactive. CD5 serves to mitigate activating signals from the BCR so that the B-1 cells can only be activated by very strong stimuli and not by normal tissue antigen. CD5 was used as a T-cell marker until monoclonal antibodies against CD3 were developed.