What is an antigen?

• A. A type of immune cell that kills infected cells
• B. Stimulates the production of an antibody
• C. A virus that causes infection
• D. A protein that divides into antibodies

Answer: (B)

An antigen is a substance that the immune system recognizes as foreign and that triggers a response, especially the production of antibodies. When an antigen enters the body, B cells respond by creating plasma cells that produce antibodies specific to that antigen. These antibodies help neutralize the invader or mark it for attack by other immune cells. So the best way to describe an antigen is that it stimulates antibody production because that is the key action that links antigens to the protective antibody response.

It's not a type of immune cell that kills infected cells, and it's not a virus itself, though a virus can contain antigens. It also isn't a protein that divides into antibodies; antibodies are produced in response to the antigen, not derived from it.