Antigenic specificity of the antigen depends on antigenic determinants or epitopes.
An epitope is defined as the immunologically active region of an immunogen that binds to antigen-specific membrane recep-tors on lymphocytes or secreted antibodies. The interaction between cells of the immune system and antigens takes place at many levels and the complexity of any antigen is mirrored by its epitope. There are two types of epitopes: B-cell epitopes and T-cell epitopes.
B-cell epitopes are antigenic determinants recognized by B cells. B-cell epitope can combine with its receptor only if the antigen molecule is in its native state. The complementary surfaces of the antibody and the antigen molecules appear to be relatively flat. Smaller molecules often fit nicely within a particular depression or groove in the antigen-binding site of the antibody molecule.
The B-cell epitope is about six or seven sugar residues or amino acids long. B-cell epitopes tend to be hydrophilic and are often located at bends in the protein structure. They are also often found in regions of proteins, which have a higher mobility; this may make it possible for an epitope to shift just a bit to fit into an almost-right site.
T cells recognize amino acids in proteins but do not recognize polysaccharide or nucleic acid antigens. This is the reason why polysaccharides are considered as T-independent antigens and proteins as T-dependent antigens. The primary sequence of amino acids in proteins determines the antigenic determi-nants recognized by T cells. Free peptides are not recognized by T cells, while the complex of MHC molecules and peptide are recognized by T cells. Thus for a T-cell response, it should recognize both the antigenic determinant and also the MHC, and therefore it is said to be MHC restricted.
In general, T-cell epitopes or antigenic determinants are small and are only 8–15 amino acids long. The antigenic deter-minants are limited to those parts of the antigen that can bind to MHC molecules. Since the MHC molecules are subjected to genetic variability, there can be difference among individuals in their T-cell response to the same stimulus. Each MHC molecule can bind several, but not all, peptides. Therefore, for a peptide to be immunogenic in a particular individual, that individual must have MHC molecules that can bind to it.