Introduction

Hemopoietic cells express a wide range of antigens on the cell membrane as well as within the cytoplasm and nucleus. Some of these antigens are widely distributed whereas others are restricted to specific cell types. The overall spectrum of antigen expression by a particular cell is defined as its immunophenotype and the technique of immunophenotyping can be applied to many areas of hematology, immunology and other clinical fields. Immunophenotyping is of paramount importance in the diagnosis of hematological malignancies but it can be used in any situation where cellular antigen expression needs to be analysed; consequently it has widespread applications within clinical and research medicine (Table 8.1). The focus of this chapter will be the application of immunophenotyping to the diagnosis of hematological malignancies.

The existence of the pluripotential stem cell is a fundamental concept in hematology. These cells possess the dual capabilities of self-renewal and multilineage differentiation (Botnick et al., 1976). Thus, a single pluripotential stem cell has the capability of producing all classes of terminally differentiated hemopoietic cell. The first step in lineage commitment occurs when this cell divides into progeny committed to either myeloid or lymphoid development. These multipotential cells can give rise, respectively, to all classes of myeloid cell (granulocytic, monocytic, erythroid and megakaryocytic) and lymphoid cell (B-, T- and natural killer (NK) cells). During this process of lineage commitment and subsequent differentiation, cells express a number of molecules such that the antigen expression profile of an individual cell is highly characteristic of a particular stage of hemopoietic differentiation. Since it is generally accepted that hematological malignancies represent the clonal proliferation of a single cell that has been ‘arrested’ at a specific stage of diVerentiation, …