antigenbindingsites

Antigen-binding sites, also called paratopes, are the portions of an antibody that recognize and bind antigens. In conventional immunoglobulins, these sites are located in the variable regions of the heavy (VH) and light (VL) chains and are formed by the pairing of one VH domain with one VL domain. The functional surface comprises six hypervariable loops known as complementarity-determining regions (CDRs 1–3 in each chain) that come together to create a specific binding pocket or groove for a particular epitope.

Antigen binding is driven by noncovalent interactions, including hydrogen bonds, ionic interactions, van der Waals forces, and hydrophobic effects. The shape and chemistry of the paratope determine specificity for a given epitope, the portion of the antigen that is bound. The paratope is complementary to the epitope in shape, charge, and hydrophobicity, enabling selective recognition amid numerous other molecules.

Antigen-binding sites are generated through genetic mechanisms: V(D)J recombination reconstructs the V region genes, junctional diversity adds variability, and somatic hypermutation during B cell responses can further alter affinity. Although most antibodies in a typical monomeric form have two identical antigen-binding sites, some antibody isotypes can form multimers with higher valency, increasing binding opportunities and avidity.

Understanding antigen-binding sites is central to antibody design, diagnostics, and immunotherapy, as binding properties determine neutralization, opsonization, and other effector functions.