1. Antigen is any foreign substance, usually a protein, that stimulates the body's immune system to produce antibodies (antibody generation).
Note: The name antigen reflects its role in stimulating an immune response - antibody generating.
2. Any foreign substance, usually a protein, that stimulates the body’s immune system to produce antibodies.
3. Any substance that stimulates an immune response by the body. The immune system recognizes such substances as being foreign, and produces cellular antibodies to fight them. Antigen/antibody response is an important part of a person's immunity to disease.
(Source: Centers for Disease Control and Prevention)