The CD79 antigen is a heterodimer consisting of two phosphoproteins designated CD79a (mb-1; 47 kDa) and CD79b (B29; 37 kDa). CD79 associates to membrane-bound Ig to create the B-cell complex allowing signal transduction to be transmitted to the interior of the cell. CD79a is specific for B-cells, appearing before the pre-B cell stage, still being present at the plasma cell stage.
CD79a is detected in the large majority of B-cell neoplasms. However, among plasma cell neoplasms, only about 50% are stained. CD79a has been reported in up to 10% cases of precursor T-cell lymphoblastic lymphoma but very rarely in mature T-cell lymphomas. In nodular lymphocyte predominant Hodgkin lymphoma, CD79a is found in of L&H cells in the large majority of cases, while in the other types, Reed-Sternberg cells are only stained in 20% of the cases. Among myeloid leukaemias, CD79a positivity has been described in type M3.
Together with CD20, CD79a is one of the most important markers for the identification of B-cell neoplasms as outlined above.
Tonsil and colon (or appendix) is recommended as positive and negative tissue controls for CD79a. In tonsil, the protocol must be calibrated to provide a distinct and strong membranous staining reaction in all mantle zone B-cells. Virtually all germinal centre B-cells must at least display a moderate and distinct staining reaction. No staining reaction must be seen in T-cells and squamous epithelial cells. In colon (or appendix), plasma cells in lamina propria must show a moderate to strong cytoplasmic staining reaction. No staining reaction should be seen in the epithelial cells.
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