Dictionary.net

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

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

Any substance which produces special antibodies by one or many injections into a susceptible animal, such as bacteria, red blood corpuscles or other foreign cells. Blood serum, milk, and bacterial extracts are also antigens, because they produce specific precipitins, but inorganic substances, and such inorganic materials as alkaloids, do not produce antagonistic bodies and are not antigens. Diphtheria and tetanus toxins, snake venoms, and ricin are antigens.[Gr.]