What is an antigen and an antibody?

• A. An antigen triggers an immune response; an antibody is a protein produced to bind and neutralize antigens.
• B. An antigen is a protein produced to fight pathogens.
• C. An antibody triggers the immune response.
• D. Antigen and antibody are interchangeable terms.

Answer: (A)

Antigens are substances that trigger an immune response; antibodies are the proteins produced by B cells that bind specifically to those antigens to neutralize them or mark them for attack. Antigens can be parts of pathogens, like viral proteins or bacterial polysaccharides, or toxins—anything the immune system recognizes as foreign. The binding between an antibody and its antigen is highly specific because the antibody fits a particular part of the antigen called an epitope. The immune response is initiated by the presence of the antigen, while antibodies act as the targeted tools that neutralize the threat and recruit other immune cells. Saying an antigen is produced to fight pathogens or that antibodies trigger the response mixes up the roles; they are distinct and not interchangeable.