Antigenbinding

Antigen binding refers to the specific interaction between an antigen and a binding molecule, most commonly an antibody (immunoglobulin) or a T-cell receptor, though other proteins and synthetic binders can also recognize antigens. The region of the binder that contacts the antigen is called the antigen-binding site; for antibodies this site is the paratope, formed by the variable domains of the heavy and light chains. The corresponding region of the antigen is the epitope.

Antibodies bind discrete epitopes with high specificity. The strength of binding, or affinity, describes how tightly a single antigen-binding site binds an epitope, while avidity describes the overall strength when multiple binding sites interact with multivalent antigens. Affinity can be enhanced by somatic hypermutation during immune responses (affinity maturation). The T-cell receptor binds peptide fragments presented on major histocompatibility complex (MHC) molecules; CD4 or CD8 co-receptors may influence signaling.

Binding can trigger immune effector mechanisms or T cell activation. Antibody binding can neutralize pathogens, promote opsonization and phagocytosis, or activate the classical complement pathway. TCR–peptide–MHC engagement can activate T cells, leading to cytotoxic activity or helper functions, often requiring additional co-stimulatory signals.

Binding kinetics are experimentally characterized by on-rate, off-rate, and equilibrium dissociation constant (Kd). Methods such as ELISA, surface plasmon resonance, and bio-layer interferometry are used to study antigen binding and to quantify affinity and specificity.

Biological and clinical relevance: antigen binding underpins immune recognition, vaccine design, diagnostics, and therapeutic antibody development. Cross-reactivity, where a binder recognizes related epitopes, can be beneficial or problematic; conformational changes and epitope masking can affect binding.