The CD5 antigen is a 67 kDa transmembrane glycoprotein expressed on the surface of practically all mature human T-cells (about 10% of CD4+ T-cells being CD5 negative). In immature (CD34+) T-cells, CD5 is weakly expressed, the intensity of expression increaing with maturation. CD5 is also expressed in a small subset of normal human B-cells (20% of B-cells in the peripheral blood, scattered cells in the lymph node mantle zone). The CD5+ cells are probably involved in B-T interaction and their ligand is CD72 which is expressed on all B cells. It appears that CD5+ B-cells on activation primarily produce IgM. They also produce more autoantibodies than normal CD5 negative B-cells. Thus, the CD5+ B-cell population is expanded in rheumatoid arthrititis and systemic lupus erythematosus.
CD5 is detected in most T-cell lymphomas and leukaemias, including 75% of peripheral T-cell lymphomas and 85% of T-ALL cases. Lack of CD5 in the latter signifies a worse prognosis. Among B-cell lymphomas, more explicit CD20+ small-cell lymphomas, small lymphocytic lymphoma and mantle cell lymphoma are CD5+, whereas follicular lymphoma, marginal zone lymphoma and lymphoplasmacytoid lymphoma are CD5 negative. CD5 is detected in 5% of acute myeloid leukaemias. CD5 has been detected in some cases of thymic carcinoma and atypical thymoma. Other carcinomas are CD5 negative.
Classification of small B-cell lymphomas, prognostication of T-ALL. Identification of large cell lymphoma (CD5 negative) supervening a CD5+ small lymphocytic lymphoma. Differentiation between reactive CD5+ T-cell infiltration and CD5 negative T-cell neoplasm. Identification of thymic carcinoma.
Normal tonsil is an appropriate control provided that at least a weak to moderate but distinct membranous staining reaction is seen in dispersed B-cells in the mantle zone of the secondary follicles in the tonsils. No staining must be seen in the germinal centre B-cells.
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