Definitions of antigen

1. any substance ( as a toxin or enzyme) that stimulates the production of antibodies

2. Substances that are recognized by the immune system and induce an immune reaction.

3. Any substance which, when introducen into the animal organism, causes the productiod of an antibody; hatpin ( 2).

4. Substances which, when injected into an animal, cause a series of physiologico- chemical changes resulting in the formation of antibodies in the blood.