Any toxin or foreign substance, usually a protein found on the surface of a pathogen, that is capable of producing an immune response in the body.
When an antigen enters the body, the immune system produces antibodies against it. Antigens are unique to a pathogen. The whooping cough bacterium, for example, will have different antigens on its surface from the TB (tuberculosis) bacterium. Different viruses also have different antigens. Antigens are used in vaccine development to elicit an immune response.
Immune System Infectious Disease Vaccines