Epitopes. In Encyclopedia of Life Sciences (ELS). (Book Chapter)

T and B lymphocytes, the mediators of adaptive immune responses, express antigen-specific receptors that allow them to discriminate between self and a virtually unlimited number of nonself molecules. Their receptors specifically recognise and bind subdomains of foreign macromolecules which are called epitopes. T-cell epitopes (TCEs) and B-cell epitopes (BCEs) differ fundamentally in the way they are recognised by the immune system. BCEs are recognised as three-dimensional structures on the surface of native antigens. TCEs are parts of internalised and processed antigens that are presented to T lymphocytes in association with molecules of the major histocompatibility complex. As in a biological system T- and B-cell receptors or antibody molecules face a virtual infinite number of structures, cognate interactions with epitopes are the basis of the primordial intelligence that drives the teleological choices of the immune system. Immunodominance directs the immune response towards selected epitopes. Key Concepts Key Concepts * Surface immunoglobulins of B cells and antibodies recognise the B-cell epitope (BCE) of an antigen in its native conformation, while T-cell receptors (TCRs) recognise the T-cell epitopes (TCEs) only after intracellular processing of the antigen, and in association with major histocompatibility complex (MHC) molecules. * The main common feature of B-cell epitopes is accessibility on the surface of the antigen. * TCE peptides that bind to the same allele of MHC molecules share common ‘anchor’ residues (motifs) that contact the MHC molecule. * TCEs presented by MHC class II molecules are longer and more variable in size and their anchor residues are less well defined than those of peptides presented by MHC class I molecules. * Patterns of anchor residues facilitate the prediction of peptide binding to MHC molecules. * The TCR–MHC–peptide tripartite cognate interaction in concert with coreceptors and other signalling modules mediate T-cell activation. * Immunodominance refers to the concept that only a small subset of the potential epitopes (TCEs or BCEs) present in a given antigen elicit an immune response. * Mimotopes are small molecules that mimic the structure of complex conformational epitopes without any sequence homology. * While TCEs can be predicted with some confidence, ill-defined characteristic features of BCEs limit their predictability. * Epitope-based vaccines will allow to better manage desired and undesired cross-reactivity.