Antibodies in the vertebrate immune system bind a diversity of foreign molecules via a highly variable binding site.

The vertebrate immune system is a highly coordinated system of interacting molecules, tissues, and processes that defend an organism against disease and infection. When a foreign pathogen invades the body, one of the immune system’s responses is to produce antibodies. These antibodies (also called immunoglobulins) are customized s that can bind to surface molecules on pathogens. A bound antibody can deactivate the pathogen, or can function like a flag, signaling to other parts of the immune system that the attached pathogen is a foreign invader and should be destroyed. Pathogens come in various shapes and sizes, ranging from viruses to microorganisms. Antibodies have to recognize and bind a diversity of pathogens in a lifetime, and the immune system can produce billions of different antibodies to accomplish this.

The surface molecules that antibodies bind to on pathogens are called antigens, and binding between antibody and antigen is highly specific. A given antibody can fit with and bind only one or a few different antigens. Each antibody is a Y-shaped molecule with two binding sites, one on each tip of the Y’s upper arms. While the majority of the molecule is similar among different antibodies, the binding sites at the tips are highly variable. Different numbers and combinations of protein building blocks (amino acids) result in different protein folding patterns at the binding site. How the proteins are folded determines the specific 3D shape and chemical characteristics at the antibody’s binding site. This binding site attaches to antigens like a lock fits a key. Many weak bonds between the binding site and the antigen enable the antibody to attach tightly.

By keeping the basic structure of the antibody consistent and only altering the binding site in response to different antigens, the immune system can produce a myriad of antibodies to combat diverse pathogens. Furthermore, antibodies can form complexes, where their multiple binding sites attach to and bring together many antigen-bearing pathogens. This helps to localize an infection and makes it easier for other parts of the immune system to respond.

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blue, yellow, red and orange microscopic image of a virus
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The yellow portions of each Y‑shaped antibody are the variable binding sites.

Last Updated April 19, 2018