Which statement best defines an antigen?

• A. A molecule that triggers an immune response and leads to antibody production.
• B. A lipid used as a solvent in staining.
• C. A carbohydrate that stores energy.
• D. A protein produced by B lymphocytes.

Answer: (C)

The essential idea is that an antigen is any molecule that the immune system can recognize and that can trigger an immune response, usually leading to antibody production. The immune system detects specific structures on antigens called epitopes, and antibodies bind those epitopes to target the invader.

In this context, an antigen is not about being a lipid solvent or a molecule with a metabolic role like storing energy, nor is it an antibody itself. An antibody is what the immune system produces in response to an antigen, so a protein made by B lymphocytes describes an antibody, not the antigen. Antigens can be proteins, carbohydrates, lipids, or nucleic acids, but what defines them is their ability to provoke an immune response and drive antibody production.