epitopes

Epitopes, also known as antigenic determinants, are the specific parts of an antigen that are recognized by the adaptive immune system. They are the molecular features that antibodies bind to or that T cell receptors recognize when presented by major histocompatibility complex (MHC) molecules. Epitopes can be part of proteins, polysaccharides, lipids, or other macromolecules, and their recognition drives humoral and cellular immune responses.

B-cell epitopes are usually accessible on the surface of an antigen and recognized by antibodies. They can be linear, consisting of a continuous stretch of amino acids, or conformational (discontinuous), formed by amino acids brought together in the folded three-dimensional structure. T-cell epitopes are peptide fragments derived from antigens that are presented by MHC class I or class II molecules to CD8+ and CD4+ T cells, respectively. Class I epitopes are typically 8–11 amino acids long and originate from intracellular proteins; class II epitopes are longer, often 13–25 amino acids, and derive from extracellular or endosomal pathways.

Epitope identification and mapping use experimental approaches such as overlapping peptide libraries and phage display, as well as computational predictions of MHC binding and processing. Epitope data guide vaccine design, diagnostics, and therapeutic antibody development. Epitope-based vaccines aim to focus immunity on protective determinants and may minimize unwanted responses.

Considerations include immunodominance, epitope variability and conservation across pathogen strains, potential cross-reactivity with host proteins, and the risk of autoimmunity. Epitope research continues to integrate structural biology, immunology, and bioinformatics to understand immune recognition and to develop targeted interventions.