What Happens When Antibodies React With Antigens?


Antibodies are the part of the human immune system that detect and destroy invading threats, such as foreign bacteria or pollen, preventing the spread of illness or disease. "Antigens" is the term used for these invading molecules, and are destroyed or rendered harmless upon reacting with the antibodies.

Lock and Key

  • On a molecular level the antibodies are shaped like a “Y,” with both tips at the top equipped with a “paratope.” This paratope is essentially a binding device, often referred to as a “lock” on the antibody. On each different antibody the paratope will be the exact shape to fit an “epitones” on an invading antigen. This slotting perfectly together has resulted in the common usage of the “lock and key” metaphor to describe the physical reaction between antibody and antigen. When scientists attempt to find an antibody for an invading antigen they are searching for the lock that the key fits.

Attacking the Antigen

  • Two things can happen when an antibody and antigen lock together. Firstly, the antibody can neutralize the invading antigen by blocking the part of the microbe which is harmful to the human body or which helps it grow. Secondly, it can identify the antigen to other parts of the human immune system.

The Cavalry Arrives

  • A good way of understanding the eradication of offending antigens is to consider the phrase “call in the cavalry.” When an antibody locks to an antigen it may not be able to able to neutralize it and is likely in such small quantities that it is vastly outnumbered by the antigens. Yet antibodies are produced by plasma cells, which quickly create further antibodies once the right “paratope” is identified, thus bringing in the cavalry.


  • Once the antigens have been eliminated, many of the antibody molecules remain in the body. These will continue to exist for years, ensuring that should the eliminated antigens come back they are once more quickly destroyed. This is the basis of immunization and vaccination, which prevents a person catching the illness in the first place. An obvious example is the flu. Each winter, children and the elderly are recommended to take the flu vaccine. This injects a small number of flu antigens, which the body’s antibodies then attack and neutralize. The remaining antibodies are then prepared to ward off further flu attacks. Because the flu virus mutates every year, each winter requires a different flu vaccine.

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  • Photo Credit Jeffrey Hamilton/Lifesize/Getty Images
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