An antigen is best described as a substance that triggers the production of antibodies.

• A. Antibody
• B. Antigen
• C. Enzyme
• D. Hormone

Answer: (B)

In the immune system, an antigen is any substance that the body recognizes as foreign and that triggers the production of antibodies. When B cells encounter an antigen, they differentiate into plasma cells that secrete antibodies specifically designed to bind that antigen. These antibodies can neutralize the invader or mark it for attack by other immune cells. Antibodies themselves are the products of this response, not the triggers. Enzymes are molecules that speed up chemical reactions, and hormones are signaling chemicals that coordinate processes in the body. So the reason this statement describes an antigen is that it names the substance that initiates the antibody-producing response.