Antigen Definition

A substance that when introduced into the body stimulates the production of an antibody. Antigens include toxins, bacteria, foreign blood cells, and the cells of transplanted organs.
 

Definition of types of antigen.

•  Immunogen - Any substance that provokes an immune response (provokes immunity) when introduced into the body. An immunogen is always a macromolecule (protein, polysaccharide). Its ability to provoke the immune response depends on its foreignness to the host, molecular size, chemical composition and heterogeneity (e.g. different amino acids in a protein).
• Tolerogen - An antigen that invokes a specific immune unresponsiveness due to its molecular form. A tolerogen can become an immunogen if its molecular form is changed.
• Allergen - An allergen is any substance that causes an allergic reaction. It can be eaten, inhaled, injected or comes into contact with skin.