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"Cytoplasmic or cell surface.

"Cytoplasmic or cell surface.

useful for leukemia classification. These include CD79 for B cells, CD3 for T cells and antimyelo-peroxidase for myeloid cells. Diagnostic panels consisting of these and other reagents, such as CD19, CD7, CD13, CD33, antiglycophorin and CD61 unequivocally classify the vast majority of cases of acute leukemia.

Leukemic cells in B-lineage ALL, the most frequent form of ALL, are typically CD19+, CD22+, CD79+ and surface Ig~ (Table 1). Most cases are CD10+ and are termed 'common' ALL, while CD 10~ cases have been called 'null' ALL. 'Common' and 'null' ALL are collectively termed 'early pre-B ALL'. Other molecules often expressed by these cells include CD24, CD34, CD72, HLA-DR and nuclear terminal deoxy-nucleotidyl transferase (TdT). In 20-30% of B-lineage ALL cases, cells express cytoplasmic mu heavy chains ('pre-B-ALL'; Table 1). The majority of pre-B ALLs are CD10+, while about 60% are CD34+ and 40% are CD20+. Rare cases in which cells express cytoplasmic and surface mu heavy chains without kappa or lambda light chains have been called 'transitional pre-B ALL'. In these cases, the surface mu heavy chains are thought to be associated with pseudo light chains. Finally, the blasts in a few (<5%) B-lineage ALL cases express surface IgM and either kappa or lambda light chains ('B-ALL'). Most B-ALLs are the leukemic phase of Burkitt's lymphoma.

In T-ALL, blasts typically express CD3 (in the cytoplasm or on the cell membrane) and CD7 (Table 1). T cell receptor proteins may be found in the cyto plasm of T-ALL blasts lacking surface TCR expression. Among other molecules, CD2, CD5, CD6 and nuclear TdT are expressed in >90% of cases while CDla, CD4, CD8, CD10 and CD21 are expressed less frequently, and HLA-DR is usually negative. Approximately 80% of T-ALL with surface TCR have the a(3 form of the receptor, the remaining cases being TCR~7Ô+.

Malignant cells in AML express myeloperoxidase, CD13 and/or CD33 (Table 2). CD34 expression is more common in cases with less differentiated morphology. HLA-DR antigens are found in most AML cases, but are negative in the majority of cases of promyelocytic leukemia (FAB M3; Table 2). CD117 is also expressed in many AML cases, with the exception of the promyelocytic and the monocytic subtypes. Leukemias whose blasts react with CD14 antibodies usually have monocytic morphology (FAB M4 and M5; Table 2). Monocytic differentiation of AML is also suggested by CDllc, and CD4 expression. These cases are usually CD34". An additional sign of monocytic differentiation is the high membrane expression of Fc receptors. Finally, cases of acute erythroid leukemia can be identified with antibodies to glycophorin A, while cases of acute megakaryocytic leukemia react with CD41b, CD42a and CD61. These subtypes are rare (<1% of acute leukemias).

Cases composed of two separate blast populations expressing either lymphoid or myeloid markers ('mixed leukemias') are extremely rare. The term 'mixed' or 'biphenotypic' leukemia has also been

Table 2 Predominant immunophenotypes according to FAB classification in AML

FAB group

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