An antigen is the specific part of a pathogen (or potential or suspected pathogen, in the case of autoimmune disease) that the immune system recognizes as a threat and produces antibodies to bind to. Any number of molecules can be an antigen, including proteins, peptides, polysaccharides, lipids, or nucleic acids. In vaccine development, choosing the appropriate antigen is crucial to invoke an adequate, lasting immune response against the pathogen. In COVID-19 vaccines, the antigen is the spike protein. A similar term, epitope, refers to the specific part of an antigen molecule that the antibody attaches itself to.
Antigens are on viruses, bacteria, fungi, tumors, parasites, and even normal cells in the body. They’re also on allergens and are the substance on the allergen molecule that causes the immune system to overreact and attack an otherwise harmless substance (such as pollen).
Deeper Dive
There are three types of antigens:
- Exogenous antigens, those from foreign substances, which are the ones most often discussed in the lay public.
- Endogenous antigens, those that exist on normal cells in your body and tell your immune system that it’s supposed to be there (leave it alone) or that it’s infected (destroy it).
- Autoantigens, the markers on cells that cause the immune syste to attack it even when it shouldn’t, the source of symptoms from autoimmune diseases.
- Tumor antigens, those on the surface of tumors, also called tumor-associated antigens (TAA), tumor-specific antigens (TSA), neoantigens or oncogenic antigens.