Antijen

An antijen (antigen) is a molecule or part of a molecule that can be recognized by the immune system and may elicit an immune response. Most antigens are proteins or polysaccharides, but small molecules called haptens are not immunogenic by themselves and require attachment to a larger carrier to become antigenic. Antigens can originate from pathogens, foods, environmental substances, or from the body itself (autoantigens).

Recognition and processing of antigens involve both B cells and T cells. B cells can recognize native antigens through their B-cell receptors and may produce antibodies specific to the antigen. T cells recognize peptide fragments that are presented on major histocompatibility complex (MHC) molecules by antigen-presenting cells, such as dendritic cells, macrophages, and B cells. CD4+ T cells interact with MHC class II–bound peptides, while CD8+ T cells interact with MHC class I–bound peptides, directing different arms of the immune response.

Classification and features of antigens include epitopes, the specific regions recognized by receptors. Epitopes can be linear or conformational. Antigenicity is the ability of a substance to be recognized by the immune system, whereas immunogenicity is its capacity to provoke a measurable immune response. Antigens can be autoantigens (from the host), alloantigens (from other individuals of the same species), or xenoantigens (from different species). Haptens become antigens only when linked to a carrier.

Clinical and practical relevance includes vaccines that use specific antigens to elicit protection, serology that detects antibodies against antigens, and transplantation or pregnancy contexts where alloantigens influence tolerance and rejection. Pathogens may also vary their antigens to escape immune detection, shaping vaccine design and immune strategies.