An epitope is a discrete region on an antigen recognized by the antigen-binding site of an antibody or the receptor on a T lymphocyte.
Structure and Immunological Role
Epitopes can consist of a short linear sequence of amino acids or a conformational surface formed by the folding of a protein. B cell epitopes are typically exposed on the surface of pathogens and can be linear or conformational, allowing antibodies to bind specifically and neutralize or opsonize the invader. T cell epitopes are peptides generated by intracellular proteolysis that are presented on major histocompatibility complex (MHC) molecules to T cell receptors; these epitopes are generally 8–11 amino acids long for MHC class I and slightly longer for MHC class II. The specificity of epitopes determines the clonal selection of lymphocytes and the magnitude of the immune response. Immunodominant epitopes elicit strong responses, whereas subdominant ones may be ignored. The binding affinity between an epitope and paratope is governed by non-covalent interactions such as hydrogen bonds and van der Waals forces. Variations in epitopes through mutation or glycosylation enable pathogens to evade immunity, contributing to antigenic drift and immune escape.
Examples and Applications
Neutralizing antibodies against viruses like SARS-CoV-2 target epitopes on the spike protein’s receptor-binding domain. In vaccine design, synthetic peptides representing conserved T cell epitopes are used to elicit cellular immunity against pathogens such as influenza and malaria. Epitope mapping identifies antibody-binding regions on allergens and autoantigens, helping diagnose hypersensitivity or autoimmune diseases. Molecular biologists use epitope tags—short peptide sequences such as FLAG or HA—fused to proteins of interest to facilitate detection with specific antibodies. Cross-reactive epitopes can lead to unintended immune responses, as seen in rheumatic fever where antibodies against Streptococcus pyogenes M protein cross-react with heart tissue. Understanding epitope structure and variability is critical for developing diagnostics, vaccines and monoclonal antibody therapies.
Epitopes are the fundamental targets for antigen recognition by the adaptive immune system. Their structural features dictate how antibodies and T cells discriminate between self and non-self and determine the success of vaccines and immunotherapies. Detailed knowledge of epitopes supports rational vaccine design and the management of immune-mediated diseases.
Related Terms: Antigen, Paratope, B cell receptor, T cell receptor, Epitope mapping