Antigens are molecules which can produce an immune response when they are detected in an organism.
- These molecules are found on the surface of cells - they show which organism they belong to.
- Foreign antigens (heteroantigens) are antigens which are not usually found in the body - they cause the immune response.
- Self antigens (autoantigens) are antigens which are recognised by the immune system, they do not stimulate an immune response.