| Distinction between T and B cells is most easily made with reference to the cells' antigen-specific receptors
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| The cells primarily involved in the immune response are the lymphocytes. In total, lymphocytes are present in the peripheral blood at between 1.5 × 109 and 3.5 ×109/L. Of these, approximately 50-70% are T cells and 10-20% are B cells. A third population termed 'natural killer' (NK) cells make up the remainder. NK cells are so called because they demonstrate the ability to kill neoplastic cells without prior exposure or sensitization. Despite their different function it is not possible to discern any morphologic features that could be used to distinguish between these lymphocyte populations. Instead, their identification is based on immunophenotypic or functional studies.
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| Each of the major populations of lymphocytes carry particular collections of markers that can assist in assign-ing their lineage. The distinction between T and B cells is most easily made with reference to their antigen receptors (Figs 36.2 and 36.3). In the B cell, the receptor is a surface immunoglobulin termed 'sIg'; it is an integral part of the cell membrane. On binding to its antigen, it brings about the cell's activation and subsequent proliferation and differentiation. In addition to sIg, B cells express several other markers, the best characterized of which include CD19, CD20 and the major histocompatibility complex (MHC) class II DR molecules. The T cell antigen receptor is termed the T-cell receptor (TCR) and it is complexed with CD3. Two other CD markers whose expression appears to be mutually exclusive on T cells are the CD4 and CD8, and they are useful in further categorizing the T cell function. NK cells are currently identified by the expression of the combination of CD16 and CD56. These markers are used in flow cytometric technology to identify cell types.
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