Molecule triggering an immune response (antibody production) in the host
An illustration that shows how antigens induce the immune system response by interacting with an antibody that matches the molecular structure of an antigen
In immunology, an antigen (Ag) is a molecule, moiety, foreign particulate matter, or an allergen, such as pollen, that can bind to a specific antibody or T-cell receptor.[1] The presence of antigens in the body may trigger an immune response.[2]
Antigens can be proteins, peptides (amino acid chains), polysaccharides (chains of simple sugars), lipids, or nucleic acids.[3][4] Antigens exist on normal cells, cancer cells, parasites, viruses, fungi, and bacteria.[1][3]
Antigens are recognized by antigen receptors, including antibodies and T-cell receptors.[3] Diverse antigen receptors are made by cells of the immune system so that each cell has a specificity for a single antigen.[3] Upon exposure to an antigen, only the lymphocytes that recognize that antigen are activated and expanded, a process known as clonal selection.[4] In most cases, antibodies are antigen-specific, meaning that an antibody can only react to and bind one specific antigen; in some instances, however, antibodies may cross-react to bind more than one antigen. The reaction between an antigen and an antibody is called the antigen-antibody reaction.
Antigen can originate either from within the body ("self-protein" or "self antigens") or from the external environment ("non-self").[2] The immune system identifies and attacks "non-self" external antigens. Antibodies usually do not react with self-antigens due to negative selection of T cells in the thymus and B cells in the bone marrow.[5] The diseases in which antibodies react with self antigens and damage the body's own cells are called autoimmune diseases.[6]
Vaccines are examples of antigens in an immunogenic form, which are intentionally administered to a recipient to induce the memory function of the adaptive immune system towards antigens of the pathogen invading that recipient. The vaccine for seasonal influenza is a common example.[7]
^ ab"Antibody". National Human Genome Research Institute, US National Institutes of Health. 2020. Retrieved 13 October 2020.
^ ab"Immune system and disorders". MedlinePlus, US National Institute of Medicine. 28 September 2020. Retrieved 13 October 2020.
^ abcd"Antigen". Cleveland Clinic. 2023. Retrieved 23 May 2023.
^ abAbbas AK, Lichtman A, Pillai S (2018). "Antibodies and Antigens". Cellular and Molecular Immunology (9th ed.). Philadelphia: Elsevier. ISBN 9780323523240. OCLC 1002110073.
^Gallucci S, Lolkema M, Matzinger P (November 1999). "Natural adjuvants: endogenous activators of dendritic cells". Nature Medicine. 5 (11): 1249–1255. doi:10.1038/15200. PMID 10545990. S2CID 29090284.
^Janeway Jr CA, Travers P, Walport M, Shlomchik MJ (2001). "Autoimmune responses are directed against self antigens". Immunobiology: The Immune System in Health and Disease (5th ed.). Elsevier España. ISBN 9780815336426. OCLC 45708106.
^"Antigenic characterization". US Centers for Disease Control and Prevention. 15 October 2019. Retrieved 13 October 2020.
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