Antigens are best described as?

• A. Foreign substances in the body that trigger an immune response
• B. Enzymes that break down proteins
• C. Receptors on lymphocytes
• D. Antibodies produced after an infection

Answer: (A)

Antigens are anything the immune system recognizes as foreign and that can provoke an immune response. They’re typically molecules or parts of molecules from invaders—like bacteria, viruses, toxins, or other foreign proteins—that the immune system can detect and respond to. An antigen serves as the target that antibodies and T cells bind to, initiating the defense. This is distinct from enzymes (which catalyze reactions), receptors on lymphocytes (which detect antigens), or antibodies themselves (which are produced to bind antigens). So the best description is a foreign substance in the body that triggers an immune response.