Introduction to human leukocytes and leukocyte antigens

The most widely used application for flow cytometry must be the identification and enumeration of human leukocyte subpopulations. This has been facilitated by the measurement of cell surface antigens by immunofluorescence labelling using monoclonal antibodies. The leukocyte lineages express specific antigens on their cell surface that allow the different subpopulations to be identified. In some cases, a single antigen may be used to identify a specific cell type; in other cases, the presence of two or more antigens may be required to characterise a particular cell. Numerous monoclonal antibodies that bind to these specific antigens have been generated and grouped together based upon their specificity.

Monoclonal antibodies and clusters of differentiation

Since 1975 when Kohler and Milstein introduced a method for generating clones of hybrid cells capable of producing monospecific immunoglobulins in high titres, a multitude of different monoclonal antibodies have been generated recognising cell surface antigens. To organise these antibodies into a classification there have been, to date, seven international workshops, the aim of which has been to group antibodies into clusters based on their cellular reactivity. These clusters have been termed clusters of differentiation or CDs. After the seventh workshop, 247 CDs were defined (see Ch. 17) and these are shown in Table 17.2. The structure and function of the antigens for many of the CDs are now known.

Surface antigen changes during hematopoiesis Multidimensional flow cytometric analysis has been used for the characterisation of hematopoietic cell differentiation and maturational pathways (Lansdorp, 1990; Loken et al., 1987a,b; Robinson et al., 1981; Terstappen et al., 1993). By the detection of gradual, coordinated changes in the expression of lineage- pecific or lineage-associated cell surface and cytoplasmic antigens, it is possible to reconstruct the maturational pathways of hemopoietic cells.