What is an antibody?

• A. A carbohydrate used for energy
• B. A lipid component of cell membranes
• C. A protein produced by lymphocytes that attaches to a specific antigen
• D. A hormone regulating metabolism

Answer: (C)

Antibodies are proteins produced by B lymphocytes that attach to a specific antigen. In the adaptive immune response, B cells become plasma cells that secrete these proteins, each with a unique binding site that matches a particular antigen. When an antibody binds its target, it can neutralize the pathogen, block its ability to enter cells, mark it for attack by other immune cells, or activate the complement system to help destroy it. This makes antibodies a key part of targeted, specific defense against invaders. They are not carbohydrates used for energy, not lipids that make up membranes, and not hormones that regulate metabolism.