ABSTRACT
Immunohistochemistry (IHC) is the process of localizing proteins in a tissue section using monoclonal (or polyclonal) antibodies directed against specific antigens (targets) and visualizing (labeling) the antibody–antigen reaction by one-step (direct) or two-step (indirect) enzymatic reactions. IHC plays an important role in diagnostic hematopathology. It is helpful in the determination of (1) cell origin, (2) degree of differentiation (maturation), and (3) prognosis. The availability of monoclonal or polyclonal antibodies, automation, and protocols for antigen retrieval designed for formalin-fixed, paraffin-embedded tissues made the immunohistochemical techniques a common practice in everyday diagnostic surgical pathology. IHC permits differentiation of non-hematopoietic from hematolymphoid tumors, but also allows for specific and detailed subclassification of tumors based on the immunophenotype, for example, T- versus B-cell lymphomas, small lymphocytic lymphoma/chronic lymphocytic leukemia (SLL/CLL) versus mantle cell lymphoma (MCL), myeloid versus lymphoblastic versus mixed phenotype leukemia, acute versus chronic leukemia, mature versus lymphoblastic lymphoma, myelodysplastic syndrome (MDS) versus MDS with excess blasts (MDS-EB) versus acute myeloid leukemia (AML), or chronic versus accelerated phase of myeloproliferative neoplasm (MPN). In bone marrow (BM) with fibrosis, when aspirate smears and flow cytometry (FC) may be not contributory, IHC helps to identify the underlying cause, for example, to differentiate between primary myelofibrosis (PMF), metastatic tumor, plasma cell myeloma (PCM), AML, classic Hodgkin lymphoma (cHL), B- or T-cell lymphoma, or hairy cell leukemia (HCL). Staining with Ki-67, markers for Epstein–Barr virus (EBV) infection, and specific immunophenotypic markers (CD30, B- and T-cell antigens, cytotoxic proteins) helps to subclassify the majority of lymphomas into WHO-defined categories [1]. Using CD10, BCL6, and MUM1 immunostaining helps to define the “cell of origin” in diffuse large B-cell lymphoma (DLBCL) and subdivide into germinal center B-cell-like (GCB-like) and activated B-cell-like (ABC-like) by applying the Hans algorithm [2]. Table 6.1 presents phenotypic markers in the major types of hematopoietic cells.
