Immunohistochemical Marker Selection in the Diagnosis and Classification of Lymphomas When lymphohematopoietic neoplasms (primarily lymphomas) cannot be readily distinguished from other undifferentiated small-cell tumors of non-lymphohematopoietic origin, an initial antibody panel that includes leukocyte common antigen (LCA/CD45), S-100, and cytokeratin (CK) should be applied to establish the tissue of origin. When the histologic findings are already sufficient to support a diagnosis of lymphoma, lineage-associated markers should be used to determine the neoplastic cell type. A practical first-line panel consists of two pan-T-cell markers (CD3 and CD43) and two pan-B-cell markers (CD20 and PAX5), together with CD15 and CD30. This combination usually permits a straightforward separation of Hodgkin lymphoma from non-Hodgkin lymphoma and, within the latter group, of B-cell from T-cell neoplasms. For lesions that display a follicular or nodular growth pattern, markers such as CD10, BCL6, CD21, CD23, BCL2, and Ki-67 are useful for delineating the relationship between the nodules and residual lymphoid follicles. When Hodgkin lymphoma is favored morphologically, the recommended panel includes CD30, CD15, LCA, PAX5, MUM1, and EBV-encoded RNA (EBER) in situ hybridization. If anaplastic large-cell lymphoma enters the differential diagnosis, EMA, ALK, and CD43 should be added. If T-cell/histiocyte-rich large B-cell lymphoma must be excluded, BOB.1, OCT2, CD20, and CD3 should be included. Small B-cell lymphomas require simultaneous consideration of small lymphocytic lymphoma/chronic lymphocytic leukemia (SLL/CLL), marginal-zone lymphoma (MZL), follicular lymphoma (FL), and mantle-cell lymphoma (MCL). An appropriate panel comprises CD20, PAX5, CD19, CD5, CD23, CD10, BCL2, cyclin D1, SOX11, LEF1, and MNDA. For neoplasms showing clear-cut immature (blastic) differentiation, the differential diagnosis includes acute lymphoblastic leukemia/lymphoblastic lymphoma (ALL/LBL) and acute myeloid leukemia (AML). In addition to lymphoblastic markers (TdT, CD99, CD10, CD34), myeloid antigens such as myeloperoxidase (MPO) and CD117 should be assessed to exclude lymphoblastic lymphoma with myeloid or bilineage differentiation. Judicious selection of these markers greatly facilitates accurate diagnosis and subclassification. In plasma-cell-rich lesions, useful markers include CD38, CD138, immunoglobulin light chains (κ and λ), MUM1, and cyclin D1. Demonstration of light-chain restriction (κ or λ) helps distinguish neoplastic from reactive plasma-cell proliferations. For lesions suspected to represent highly aggressive mature B-cell neoplasms—including the majority of diffuse large B-cell lymphomas, Burkitt lymphoma, B-cell lymphomas with features intermediate between these two entities, high-grade B-cell lymphoma, and high-grade follicular lymphoma—the recommended panel includes CD10, BCL6, BCL2, MUM1, MYC, CD5, and p53, supplemented by EBER in situ hybridization. In diagnostically difficult cases, additional markers, sometimes applied in successive rounds, may be required; however, restraint in panel size remains important. Because of known antigenic cross-reactivity, two independent T-cell markers and two independent B-cell markers are advisable to provide more reliable evidence of lineage. Combined morphologic evaluation and immunophenotyping allow accurate diagnosis and classification of the great majority of lymphomas. Molecular studies. When the above investigations still leave the diagnosis unresolved, ancillary molecular techniques—including antigen-receptor gene-rearrangement analysis, fluorescence in situ hybridization (FISH), and gene-expression profiling—should be employed.