antigenicity

Antigenicity is the property of a molecule to be recognized by components of the immune system, most notably antibodies and T cell receptors. An antigenic molecule contains epitopes, specific structural features that bind to immune receptors. Antigenicity describes recognition, while immunogenicity refers to the ability to provoke an immune response; a substance can be antigenic without being strongly immunogenic in a given context (for example, haptens require a carrier to become immunogenic).

Determinants of antigenicity include the nature and arrangement of epitopes, whether they are linear or conformational, and the overall three-dimensional structure of the molecule. Other factors such as size, multivalency, post-translational modifications (like glycosylation), and the way an antigen is presented or exposed to the immune system also influence antigenicity. Haptens are small molecules that are antigenic only when covalently attached to a larger carrier protein. Antigenic molecules can be proteins, peptides, polysaccharides, lipids, or other macromolecules; polysaccharides are often antigenic in the context of B-cell responses.

In practice, antigenicity underpins serological diagnostics and vaccine design. Accurate antigen recognition is essential for diagnostic tests that detect antigens or antibodies. In vaccines, antigens are chosen for their ability to be recognized by the immune system and, ideally, to elicit robust and protective responses. Cross-reactivity between similar epitopes can lead to false positives, and antigenic variation or drift can alter epitopes and affect immune recognition. Epitopes recognized by antibodies are termed B-cell epitopes, while those presented to T cells are processed and displayed by MHC molecules as T-cell epitopes.