Such molecules are referred to as cluster of differentiation (CD) markers. The CD classification was established in 1982; while originally intended only for white blood cell markers, its use has since expanded to many other cell types. At last count, over 350 CD antigens have been identified. It is now understood that each cell type has its own distinct markers that can serve as identification tags. Many of these CD markers have helped physicians to understand why there is so much variation among the different types of lymphoma.
Different classes of cells can readily be distinguished, however, by virtue of the fact that they express unique combinations of molecules in their membranes. Our knowledge of the different cell types involved in immune responses is a direct result of the development of reagents to distinguish these various cells by their cell surface markers. We will see that such markers include molecules distinguished either by antibodies directed against them, or else by their ability to bind various other molecules or cells.
As both B- and T-lymphocytes mature, they go through several stages of development. During these stages,lymphocytes will acquire new CD markers, while other CD markers will diminish in their expression or be lost altogether.
Also associated with the TcR is a complex of proteins known as CD3, which participate in the transduction of an intracellular signal following TcR binding to its cognate MHC/antigen complex.
T-cells consist of a variety of subpopulations, each of which can carry out one or more specific immune functions. One important way in which these subpopulations can be distinguished is by the use of the two cell surface markers CD4 and CD8.
Immunochemical staining of CD4 in human lymphoid node with mMAb
There have been significant advances in the identification and characterization of the cell surface molecules that are expressed on normal lymphoid and myeloid cells. The application of these markers to the study of leukemias and lymphomas has initiated an understanding of the heterogeneity and both the biologic and clinical behavior of these tumors. It is now possible to assign a lineage derivation to a neoplastic hematopoietic cell in virtually all cases by the expression of lineage-restricted antigens. More importantly, by using markers that define stages of normal T, B, and myeloid differentiation, it is possible to begin to identify subsets of patients within histologically defined subgroups of leukemias and lymphomas that demonstrate unique clinical presentations, disease courses, and responses to therapy.
Basic immunophenotypic schematic for primary cutaneous T-cell and B-cell lymphomas
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