T CELLS AND THEIR RECEPTORS

Each T cell is also committed to a given antigen and recognizes it by one of two TCRs. They may have TCR2s composed of gamma (γ) and delta (δ) chains or TCR2s composed of another heterodimer of alpha (α) and beta (β) chains. These TCR2s are associated with a group of transmembrance proteins on the CD3 molecule, which takes the antigen recognition signal inside the cell. Signal transduction via the CD3 complex is regulated by a series of kinases, which are associated with the tails of the CD3–TCR complex and regulate phosphorylation. Defi ciencies or blocks in the T-cell signaling pathways either at the cell-surface complex or at the level of the kinases may result in various forms of immunodefi ciency. Two other important antigens present on TCR2 cells recognize histocompatibility antigens and will be discussed later. The genes for TCR chains are on different chromosomes with the β and α molecules on chromosome 7, while the α and δ are on chromosome 14. As seen in Figure 1.5, the four chains are made up of a variable region and a constant region similar to those observed with the immunoglobulins. The variable regions are also numerous and joined at D and J regions by RAG1 and RAG2. This permits a diversity of antigen recognition similar to that observed with immunoglobulin, but additional somatic mutation is not involved in T cells. These similarities have led to the concept that genes for antigen-specifi c T cells evolved in the same manner as immunoglobulin from a parent gene, and both are members of a superantigen family. 

The TCR complex recognizes small peptides presented to it by the MHC class I and II and depends on the type of T cell.  Helper T cells (CD4) recognize class II antigens while suppressor cytotoxic T cells (CD8) recognize class I antigens. Because of the rather low affi nity of the reactions, recognition of processed antigen alone is not suffi cient to activate T cells. Soluble interleukins are needed to complete the picture and are generated during the antigen processing.