CD5 (protein)

CD5 is a is_associated_with::cluster of differentiation found on a subset of is_associated_with::IgM-secreting is_associated_with::B cells called is_associated_with::B-1 cells, and also on is_associated_with::T cells. B-1 cells have limited diversity of their is_associated_with::B-cell receptor due to their lack of the enzyme is_associated_with::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 (such as bacterial proteins) and not by normal tissue proteins. CD5 was used as a T-cell marker until is_associated_with::monoclonal antibodies against CD3 were developed.

In humans, the gene is located on the long arm of is_associated_with::chromosome 11. There is no confirmed ligand for CD5 but there is evidence that CD72, a C-type lectin, may be a ligand or that CD5 may be homophilic, binding CD5 on the surface of other cells.

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

Immunohistochemistry
CD5 is a good immunohistochemical marker for T-cells, although not as sensitive as CD3. About 76% of T-cell is_associated_with::neoplasms are reported to express CD5, and it is also found in is_associated_with::chronic lymphocytic leukemia and is_associated_with::mantle cell lymphoma cells. It is commonly lost in is_associated_with::cutaneous T-cell lymphoma, and its absence can be used as an indicator of malignancy in this condition. The absence of CD5 in T cell is_associated_with::acute lymphoblastic leukemia, while relatively rare, is associated with a poor prognosis.