T cell activation through the antigen receptor (TCR) involves the cytoplasmic tails of the CD3 subunits CD3 gamma, CD3 delta, CD3 epsilon and CD3 zeta. These CD3 subunits are structurally related members of the immunoglobulins super family encoded by closely linked genes on human chromosome 11. The CD3 components have long cytoplasmic tails that associate with cytoplasmic signal transduction molecules. This association is mediated at least in part by a double tyrosine-based motif present in a single copy in the CD3 subunits. CD3 may play a role in TCR-induced growth arrest, cell survival and proliferation.. The CD4 antigen is involved in the recognition of the type II MHC antigen. It is also a receptor for HIV. It is present on most T helper cells and normal thymocytes. The cytoplasmic portion of CD4 is associated with p56lck tyrosine kinase. CD4 expression is commonly found in human lymph nodes and tonsils.. CD8 molecule is composed of two chains termed alpha and beta. CD8 is found on a T cell subset of normal cytotoxic / suppressor cells which make up approximately 20 to 35 % of human peripheral blood lymphocytes. The CD8 antigen is also detected on natural killer cells, 80% of thymocytes, on a subpopulation of 30% of peripheral blood null cells and 15 to 30% of bone marrow cells.
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