Antygen

An antigen, a term often used as antygen in Polish, is a molecule or a part of a molecule that can be recognized by the immune system and evoke a response by binding to antibodies or to receptors on B cells and T cells. Antigens are usually foreign to the host, such as components of pathogens, environmental substances, or transplanted tissues, but self-molecules can also act as antigens in the context of autoimmunity.

Antigenicity refers to the ability of a substance to be recognized by the immune system, whereas immunogenicity is the ability to trigger an immune response. These properties depend on factors such as size, complexity, stability, route of exposure, and the capacity to be processed and presented by antigen-presenting cells. An antigen may contain multiple epitopes, which are the specific parts recognized by antibodies or T-cell receptors. Epitopes can be linear or conformational, and some molecules contain several distinct epitopes.

Processing and presentation are central to adaptive immunity. Exogenous antigens are taken up by antigen-presenting cells, processed into peptides, and displayed on MHC class II molecules to CD4+ T helper cells. Endogenous antigens, including viral or tumor proteins, are processed and presented on MHC class I molecules to CD8+ cytotoxic T cells. B cells can recognize native antigens and, with help from T cells, differentiate into plasma cells that produce antibodies.

Types of antigens include exogenous, endogenous, autoantigens, and alloantigens such as different MHC variants between individuals. Haptens are small molecules that become immunogenic when bound to larger carrier proteins. Understanding antigens underpins vaccines, allergies, and autoimmunity.