plicable to frozen tissue, immunohistochemistry can often be applied to paraffin-embedded tissue. Moreover, the technique is generally available at present in most pathology departments and can be used on archival material as new antibody reagents are developed, provided the antibody recognizes antigens that are not destroyed by paraffin embedding.

Immunohistochemistry is frequently helpful in determining the lineage of tumors and can be used to evaluate the clonality of many B lymphocyte proliferations, as well as to generate useful information regarding the malignant or benign nature of T cell proliferations (15,16). As such, it is particularly important in the diagnosis of lymphomas and leukemias, with several caveats: (1) aberrant expression of myeloid antigens on lymphoid neoplasms or vice versa—a phenomenon variously referred to as lineage infidelity (17), heterogeneity (18), or promiscuity (19,20)—is seen in a significant minority of acute leukemias, making precise assignment of lineage difficult in this subset of cases; and (2) direct assessment of clonality in T cell proliferations with immunohistochemical reagents is not feasible at present.

Immunohistochemistry can also be valuable for determining the tissue of origin of solid tumors and can provide prognostic information in certain cases. The principal limitation at present is that most genetic changes associated with oncogenesis cannot be assayed by using this technology.