Major function of T-lymphocytes is to recognize surface structures of organism's own cells. If there is something on cell surfaces "to irritate" T-lymphocyte (for instance, admixture of viral peptides), it tries and organizes elimination of defective cell.
• In contrast to B-lymphocytes T-lymphocytes do not produce soluble forms of Ag-recognizing molecules and always "work" by means of own "cellular body". Moreover most lymphocytes are unable to recognize and bind soluble Ag.
• In order to "attract attention" of T-lymphocyte to Ag other cells should make Ag pass through them by some way or another. They present the processed Ag on the cell membrane in complex with MHC-I/II. This is the phenomenon of Ag presentation to T-lymphocyte. Recognition of the complex by T-lymphocyte is called double recognition or MHC-restric-tion of T-lymphocytes.
T-CELL ANTIGEN RECEPTOR (TCR)
Antigen-recognizing Rc of T-lymphocytes - TCR belongs to immunoglobulin superfamily (see Fig. 5.1). Ag-recognizing site of TCR prominent over cell surface is heterodimer (i.e. consists of two different polypeptide chains) - an analogue of one Fab-fragment of Ig. Two TCR variants are known and designated as TCRap and TCRy8; the variants differ by composition of polypeptide chains of Ag-recognizing site. Each T-lymphocyte bears only one Rc variant. Tap are known and examined prior and in detail as compared to Ty8. Hence the structure of T-lymphocyte Rc for Ag is to be described from TCRap. TCR is a transmembrane molecule and consists of 8 or 10 (one or two pairs of a + p and complex "2e + 8 + у +2Z") polypeptide chains (Fig. 6.1).
• TCR transmembrane a and p chains
♦ They are nearly equal polypeptide chains - a (molecular mass of 40-60 kDa, acid glycoprotein) and p (molecular mass of 40-50 kDa neutral and basic glycoprotein). Each chain comprises 2 glycosylated domains in Rc intracellular part, hydrophobic (positive load by lysine and arginine residues) transmembrane region and short (5-12 amino acid residues) cytoplasmic site. Extracellular parts of both chains are linked by one disulphide bond.