CD5 antigen is usually expressed on the surface of practically all mature peripheral human T cells, but only weakly expressed in immature T cells. Therefore, CD5 is detected by IHC in most T cell lymphomas and leukemias, including 75% of peripheral T cell lymphoma and 85% of T lymphoblastic lymphoma/leukemia. CD5 is also expressed in subset of normal human B cells. CD20+ small B cell lymphomas can be identified and subclassified by CD5 expression. Small lymphocytic lymphoma/leukemia and mantle cell lymphoma are CD5 positive, whereas follicular lymphoma, marginal zone lymphoma and lymphoplasmacytic lymphoma are CD5 negative. CD5 IHC can be very useful in the identification of thymic carcinoma from thymoma since CD5 is expressed on the surface of thymic carcinoma cells.