An antigen is any substance that the immune system recognizes as foreign (non‑self) and that can trigger an immune response. Antigens are often proteins or polysaccharides on the surfaces of bacteria, fungi, viruses or other cells【995018077267007†L119-L127】.
Explanation
The immune system distinguishes between the body’s own molecules and those that originate from pathogens or other sources. Structures that are not recognized as self are called antigens. When antigens bind to receptors on B cells or T cells, they stimulate the production of antibodies or the activation of cytotoxic responses. Typical antigens include surface proteins and polysaccharides of bacteria, fungi and viruses【995018077267007†L119-L127】, toxins, components of transplanted tissues and environmental allergens. Antigens are processed by antigen‑presenting cells and displayed to helper T cells via major histocompatibility complex (MHC) molecules. B cells can also respond directly to soluble antigens, differentiating into plasma cells that secrete antibodies specific for the antigenic epitope. Memory cells generated during this process enable rapid recognition of the same antigen upon subsequent exposure【995018077267007†L119-L133】. Self proteins generally do not trigger a response, but breakdowns in tolerance can result in autoimmunity, where the immune system mistakenly reacts against the body’s own antigens【995018077267007†L119-L133】.
Examples and applications
Bacterial lipopolysaccharide, flagellin and capsule polysaccharides are examples of antigenic molecules that elicit strong innate and adaptive responses. Viral antigens include capsid proteins and envelope glycoproteins, while fungal antigens encompass surface mannoproteins and glucans. Environmental antigens such as pollen and peanut proteins act as allergens in susceptible individuals, triggering hypersensitivity reactions. Blood group antigens (A, B, Rh) on erythrocytes determine compatibility for transfusions, and mismatch can lead to hemolytic reactions. Vaccines use weakened pathogens or purified antigens to train the immune system to recognize specific microbes without causing disease.
Understanding antigens and how they are recognized lies at the heart of immunology and vaccine development.
Related Terms: Antibody, Epitope, Immunogen, Vaccine, Major histocompatibility complex