Antigens

An antigen is any substance that can elicit an immune response. Most antigens are proteins or polysaccharides, though lipids and nucleic acids can be antigenic under certain conditions. An antigen typically binds to a specific antibody or to a T cell receptor, and the portion recognized by the receptor is called an epitope. A single antigen may contain multiple epitopes that engage different receptors.

Not all antigens are immunogenic; immunogenicity is the ability to provoke an immune response. Factors include size, structural complexity, foreignness, stability, and the context of exposure. Haptens are small molecules that are not immunogenic by themselves but can become immunogenic when linked to a larger carrier protein.

Antigen processing and presentation involve two main pathways. Exogenous antigens from outside the body are taken up by antigen-presenting cells and processed into peptides displayed on MHC class II molecules to CD4+ T helper cells. Endogenous antigens, produced within cells (such as viral or tumor antigens), are processed and presented on MHC class I molecules to CD8+ cytotoxic T cells. Some antigens can be presented by both pathways (cross-presentation).

Antigen types include exogenous antigens (microbes and environmental substances), endogenous antigens (originating inside cells), and autoantigens (self-derived, which can contribute to autoimmunity). Allergens are a subset of antigens that trigger hypersensitivity reactions in susceptible individuals.

Clinical and practical relevance centers on vaccines that introduce antigens to provoke protective immunity, diagnostic tests that detect antigen- or antibody-based responses, and immunotherapies targeting specific antigenic targets. Antigenic variation and molecular mimicry can influence disease outcomes and immune recognition.