Читать книгу Immunology - Richard Coico - Страница 116

An immune response induced by an antigen generates antibodies or lymphocytes that react specifically with the antigen. The antigen‐binding site of an antibody or a receptor on a lymphocyte has a unique structure that allows a complementary fit to some structural aspect of the specific antigen. The portion of the immunoglobulin that specifically binds to the antigenic determinant or epitope is concentrated in several hypervariable regions of the molecule, which form the complementarity‐determining region (CDR). Additional structural features of the immunoglobulin molecule are described in Chapter 6.

Various studies indicate that the size of an epitope that combines with the CDR on a given antibody is approximately equivalent to 5–7 amino acids. These dimensions were calculated from experiments that involved the binding of antibodies to polysaccharides and to peptide epitopes. Such dimensions would also be expected to correspond roughly to the size of the complementary antibody‐combining site (termed paratope), and indeed this expectation has been confirmed by X‐ray crystallography. The small size of an epitope (peptide) that binds to a specific T‐cell receptor (TCR) (peptides with 8–12 amino acids) is made functionally larger, since it is noncovalently associated with MHC proteins of the antigen‐presenting cell. This bimolecular epitope–MHC complex then binds to the TCR, forming a trimolecular complex (TCR–epitope–MHC).