An antigen is any substance that the immune system recognizes as foreign and can provoke an immune response. Antigens are typically proteins or polysaccharides on the surface of pathogens like bacteria and viruses.
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Antigens are recognized by specific receptors on B cells and T cells.
The part of the antigen that is recognized by the immune system is called an epitope.
Antigens can be exogenous (originating outside the body) or endogenous (originating inside the body).
Vaccines work by introducing antigens to stimulate an immune response without causing disease.
Autoantigens are normal body proteins that are mistakenly targeted by the immune system in autoimmune diseases.