An antigen is a substance that is perceived as being foreign or dangerous by the body’s immune system. The immune system fights it by producing antibodies. Antigen binds to specialized protein on outer surface of plasma cell (a type of white blood cell) and stimulates it to produce large quantities of antibodies against it. The potency of binding an antibody and antigen is known as affinity.
Antigens can be microorganisms (bacteria, viruses, and fungi), pollen, toxins, foreign blood cells or some chemicals. In pathological condition like lupus and other autoimmune diseases, normal body proteins are detected as antigens, which causes the activation of antibodies and thereby destroy the healthy proteins of the body.