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of classification of the various neoplasms (42,43). Immuno-phenotypic studies are an integral part of lymphoma diagnosis, as clearly stated in the recent WHO classification of hema-tolymphoid tumors. Immunophenotype can be determined by flow cytometric analysis if fresh tissue is available or by IHC on routinely processed tissue. Because of its quantitative properties, the first technique can be invaluable in finding subtle clonal B-cell populations and detecting aberrant T-cell antigen expression. In addition, some markers that are detected on the fresh cells used for flow cytometry (e.g., the B-cell marker CD19) are not yet available for use on formalin-fixed tissue. However, more and more, lymphoid markers are becoming available for use in paraffin-embedded tissue. The unique ability of IHC to combine morphology and immunophenotype is one reason that it is currently the method of choice compared to other tests such as flow cytometry and molecular studies in the evaluation of lymphoid neoplasms.

Some B-cell lymphomas can be accurately diagnosed based on morphologic grounds, but the presence of morphologic overlap between different entities usually requires IHC studies using a variety of markers. CD20 is a reliable B-cell marker with membranous staining that is positive in the large majority of small and large B-cell lymphomas. Pax-5, a more recently introduced B-cell marker, is localized in the nucleus of normal and neoplastic B-cells. It is positive in B-cell lymphomas (including precursor B-cell lymphoblastic lymphoma, which is usually negative for CD20) as well as in Hodgkin lymphoma (44). Although this marker is gaining in popularity, the extensive experience with staining and interpretation of CD20 as well as the lower cost make it the antigen of choice for characterizing lymphoid cells as B-cells. In some special circumstances, such as patients receiving anti-CD20 therapy (rituximab), Pax-5, or CD79a can help demonstrate B-cell lineage. Pax-5 is also of value in demonstrating B-lineage of precursor B-cell neoplasms and in distinguishing Hodgkin lymphoma from anaplastic large-cell lymphoma. The antiapop-tosis protein bcl-2 is a marker that distinguishes follicular hyperplasia (bcl-2-negative) from follicular lymphoma that typically overexpresses bcl-2 (45). It is a common misconception that the presence of bcl-2-positivity supports that a lymphoma is of follicle center origin. This is not true, because a variety of neoplastic and non-neoplastic lymphoid cells express this antigen.

Table 2

IHC Panel in Small B-Cell Lymphomas

Table 2

IHC Panel in Small B-Cell Lymphomas

Lymphoma

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