CD5 (protein)

CD5 is a cluster of differentiation found on a subset of IgM secreting B cells called B-1 cells, and also on T cells. B1 B cells have limited diversity of their B Cell Receptor (BCR) 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 B1 cells can only be activated by a very strong stimuli (such as bacterial proteins) and not by normal tissue proteins. CD5 was used as a T cell marker until mABs against CD3 were developed. In humans, the gene is located on the q arm of chromosome 11. CD72 serves as a ligand.

T cells express higher levels of CD5 than B cells. CD5 is up-regulated on T cells upon strong activation. In the thymus, there is a correlation with CD5 expression and strength of the interaction of the T cell towards self-peptides.