An antigen is a substance that triggers the immune response, causing the body to form and produce specific antibodies.
How antigens work: 1) The invading organism with antigen A and antigen B on its cell surface; 2) Antibodies A and B are produced in response to antigens A and B, respectively; and 3) The specific antibodies bind with the specrfic corresponding antigens, rendering them harmless. (From Smeltzer, S.C., and Bare, B.G. Textbook of Medical-Surgical Nursing. 9th Ed. Philadelphia: Lippincott, Williams & Wilkins, 2000.)