protein_function: May act as a receptor in regulating T-cellproliferation.
CD5, also known as T1 or LEU1, is a cluster of differentiation found on a subset of IgM-secreting B cells called B-1 cells, and also on T cells. It is mapped to 11q12.2. Although CD5 can be found on both T cells and B cells, T cells express higher levels of CD5 than B cells. CD5 was used as a T-cell marker until monoclonal antibodies against CD3 were developed. CD5 is upregulated 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. Nevertheless, CD5 serves to mitigate activating signals from the BCR so that the B-1 cells can only be activated by very strong stimuli(such as bacterial proteins) and not by normal tissue proteins.