CD79a (MB1) is a B-cell marker with a wider range of positivity in the B-cell development spectrum compared to CD20 (mature B-cell phenotype). CD79a may be expressed on malignant precursor B-cells and terminally differentiated B-cells (PAX-5 is not expressed in terminally differentiated B-cells). Please note that in the setting of ALL, CD79a is not lineage specific, and up to 50% of T-ALL cases express CD79a. AML cases with t(8;21) may also show CD79a expression (along with CD19, CD20, and TdT).
Rituximab Therapy
Rituximab (anti-CD20 antibody) used for treatment of many B-cell lymphoproliferative disorders may result in the down-regulation of CD20 expression. Therefore, alternative B-cell markers need to be used to identify neoplastic B-cells. CD79a is such an antibody. Additional useful B-cell markers include PAX-5 and CD19. Some studies have showed down-regulation of multiple B-cell markers in the setting of rituximab use, and therefore a panel of B-cell markers may be necessary.
Follicular Lymphoma
CD79a has been reported to have a different expression pattern in follicular lymphoma (FL) in comparison to reactive germinal centers. Normally, reactive germinal center B-cells show dimmer expression compared to the surrounding mantle zones. In FL the neoplastic follicles show strong expression in both the mantle zone and the neoplastic follicles.
CD20-negative B-cell Lymphomas treated with rituximab (~84%)
Photomicrographs
Moderate expression of CD79a in a B-cell lymphoma.Differential expression of CD79a in the reactive germinal center (dimmer) compared to the adjacent mantle zone in a tonsil.
References
Boyd SD, Natkunam Y, Allen JR, Warnke RA. Selective immunophenotyping for diagnosis of B-cell neoplasms: immunohistochemistry and flow cytometry strategies and results. Appl Immunohistochem Mol Morphol. 2013;21: 116–131. doi:10.1097/PAI.0b013e31825d550a
MD DY-PW, BacSc F. A case of t (14; 18)-negative follicular lymphoma with atypical immunophenotype: usefulness of immunoarchitecture of Ki67, CD79a and follicular dendritic cell …. … Malaysian journal of …. 2014.
Maeshima AM, Taniguchi H, Fukuhara S, Morikawa N, Munakata W, Maruyama D, et al. Follow-up data of 10 patients with B-cell non-Hodgkin lymphoma with a CD20-negative phenotypic change after rituximab-containing therapy. Am J Surg Pathol. 2013;37: 563–570. doi:10.1097/PAS.0b013e3182759008
Chu PG, Loera S, Huang Q, Weiss LM. Lineage determination of CD20- B-Cell neoplasms: an immunohistochemical study. Am J Clin Pathol. 2006;126: 534–544. doi:10.1309/3WG32YRAMQ7RB9D4
