Antigen

What is Antigen?

A molecule, usually on the surface of a pathogen, that is recognised as foreign and stimulates a specific immune response. Different pathogens carry antigens of different shapes, which is why the antibody produced against one pathogen does not act against another.

This definition is part of the Disease and Immunity chapter in Cambridge O Level Biology.

Concepts related to Antigen

• Antibody
• lymphocyte
• vaccine

Common mistakes with Antigen

• "Antibodies engulf pathogens." Antibodies are proteins. A protein cannot surround and swallow anything. Phagocytes are cells, and they engulf. Write Antibodies bind to antigens, which can clump pathogens together or mark them so that phagocytes engulf and digest them.
• "Antigens are made by lymphocytes." Antigens belong to the pathogen. Lymphocytes make antibodies against them. Write An antigen is a molecule, usually on the pathogen's surface, that stimulates a specific immune response.
• "Vaccination gives passive immunity." A vaccine supplies antigens, not antibodies, and your own lymphocytes do the work. Write Vaccination stimulates active immunity: the person's own lymphocytes produce antibodies and memory cells are formed.
• "Vaccines always contain live pathogens." They may contain weakened pathogens, inactivated pathogens, or the antigens alone. Write A vaccine contains antigens from the pathogen, in a form that does not cause the disease.
• "Passive immunity produces memory cells." The recipient's own lymphocytes never met the antigen, so there is nothing to remember. Write Passive immunity gives immediate protection but no memory cells are formed, so protection is short-term.

Questions students ask about Antigen