Introduction

The diagnosis and treatment of childhood leukemia rest on the recognition of a leukemic cell population and its cell lineage and, sometimes, the stage of maturation. The presence in leukemic blasts of myeloperoxidase, Auer rods, or monocyte-associated esterases readily identify most cases of acute myeloid leukemia (AML). By contrast, the leukemic blasts of acute lymphoblastic leukemia (ALL) have no unique morphologic or cytochemical features. Malignant megakaryoblasts also lack defining cytologic and cytochemical features and may be mistaken for ALL. Although rare in children, chronic lymphoid malignancies, such as large granular lymphocyte leukemia or HTLV-1-associated leukemia can be confused with reactive lymphocytosis or acute leukemia. The prognosis and therapy for ALL, AML, and chronic leukemias differ greatly; thus, it is crucial to document the lineage and stage of maturation of leukemias. In the absence of diagnostic morphologic features, accurate diagnosis requires contemporary immunologic and molecular analyses. Immunologic testing, or immunophenotyping, is an essential component of the diagnostic work-up by confirming or establishing the leukemic cell lineage, stage of differentiation, and sometimes clonality. The results of immunophenotyping also correlate with cytogenetic abnormalities, facilitate minimal residual disease studies, and provide prognostic information.

The earliest immunophenotyping studies of leukemias were performed with polyclonal antisera produced to T lymphocytes, immunoglobulin (Ig) heavy and light chains, the common acute lymphoblastic leukemia antigen (CALLA), the minor histocompatibility antigen HLA-DR, and terminal deoxynucleotidyl transferase (TDT).