What Is an Antigen & an Antibody?


Both antigens and antibodies are part of the human body's immune system. Antigens are usually large, complex foreign substances that cause the production of antibodies. A substance that has an antigen on the surface is antigenic. The body recognizes these foreign antigens as invaders and moves to destroy them with lymphocytes, or white blood cells, which secrete antibodies. Antigens may be found on the surface of things such as foods, red blood cells, cancer cells, and pollen. The body produces antibodies for the sole purpose of destroying antigens.


  • Prior to birth, your lymphocytes learn which cells belong as part of your body, and see those as non-antigenic or safe. Your immune system recognizes "you" and does not try to destroy those cells. This is called immunotolerance. Any new substance noted by the immune system will be considered an invader and destroyed.


  • Antibodies are secreted by the body's B-lymphocytes, a type of white blood cell. When these cells recognize an invader to the body, they increase the secretion of antibodies. Blood and other tissue fluid carry antibodies throughout the body to where they are most needed. Antibodies attack antigens in two ways: indirectly and directly.

Indirect Attack of Antibodies

  • The most effective way for antibodies to destroy antigens is by activating complement proteins to help attack the antigens. These proteins can do a variety of things. They can rupture the invading cells, promote clumping of the cells, or weaken the foreign antigens. These proteins react in the way that is most efficient for dealing with the antigen that is recognized.

Direct Attack of Antibodies

  • Antibodies also attack antigens by directly binding to or attacking the membrane of an antigen. This physical reaction, called an antigen-antibody reaction, causes the cells to clump together. This agglutination makes it easier for other white blood cells to destroy the invading antigen. This is not as effective as the indirect route.

Primary and Secondary Response

  • The primary response is the initial response of the body to an antigen. Usually this results in a low number of antibodies produced. This is why it takes days to fight off a disease and to feel better. It is a slow process. If the immune system is exposed to the same antigen a second time, it reacts in a much more forceful way, producing large numbers of antigens quickly. This secondary response is why a person does not usually get the same disease twice and is what allows you to become immune to certain diseases.


  • A vaccine is a substance, introduced into the body, which contains antigens of a particular type. Usually the antigen used is a weakened or killed version of a disease. The purpose of this is to stimulate antibody formation. These antibodies produce memory cells that will remember how to fight that particular invader. This will then allow the body to be protected from that disease in the future, using the secondary response of the antibodies.

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  • The Human Body in Health and Illness; Barbara Herlihy, PhD, RN; 2003
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