T cells recognize antigen via the T-cell receptor, a two-chain receptor which is homologous to Ig and has constant and variable regions. Unlike antibody, the T-cell receptor serves only antigen recognition and has no intrinsic effector function; thus its constant region is much simpler than that of Ig. The basic construction and genetics of the T-cell receptor are very similar to Ig; it too has three CDRs, the most variable of which is CDR3 which probably comes into contact with the antigen molecule.
However, there is a fundamental difference between T and B antigen recognition, since T cells interact only with 'processed' antigen bound to a major histocompatibility complex (MHC) molecule on a second cell. Therefore T-cell activation requires 'antigen presentation'; consequently T cells predominantly recognize the primary sequence of antigenic peptides and are insensitive to native conformation.
There are two principal types of antigen-presenting MHC molecule: class I and class II. Each MHC molecule contains a peptide-binding site which is capable of binding a large number of peptides, each predominantly 10 to 20 amino acids long. The antigen-binding region of the T-cell receptor interacts with the exposed surface of the peptide bound within the MHC binding site, and also with the sides of the binding site themselves.
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