Antibodies Bind a Diversity of Molecules

Blue, yellow, red and orange microscopic image of a virus

Biological Strategy

Vertebrates (Mammals, Fish, Birds, Reptiles)

Jeanette Lim

Image: National Institute of Allergy and Infectious Diseases, National Institutes of Health (NIH) / Flickr / Some rights reserved

Attach Temporarily

Living systems must sometimes, temporarily, stay in one place, climb or otherwise move around, or hold things together. This entails attaching temporarily with the ability to release, which minimizes energy and material use. Some living systems repeatedly attach, detach, and reattach for an extended time, such as over their lifetimes. Despite being temporary, these attachments must withstand physical and other forces until they have achieved their purpose. Therefore, living systems have adapted attachment mechanisms optimized for the amount of time or number of times they must be used. An example is the gecko, which climbs walls by attaching its toes for less than a second. Other examples include insects that attach their eggs to a leaf until they hatch, and insects whose wings temporarily attach during flight but separate after landing.

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Protect From Microbes

In living systems, microbes play important roles, such as breaking down organic matter and maintaining personal and system health. But they also pose threats. Bacteria can be pathogens that cause diseases. Some bacteria create colonies called biofilms that can coat surfaces, reducing their effectiveness–for example, inhibiting a leaf’s ability to photosynthesize. Living systems must have strategies for protecting from microbes that cause disease or become so numerous that they create an imbalance in the system. At the same time, living systems must continue living in harmony with other microbes. Some living systems kill microbes. Others repel without killing to reduce the chances that microbes will adapt to the lethal strategy and become resistant to it. For example, some pea seedlings exude a chemical that inhibits biofilm buildup.

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Vertebrates (Mammals, Fish, Birds, Reptiles)

Subphylum Vertebrata (“jointed”): Mammals, fish, birds, reptiles

Vertebrates are a subgroup of chordates, which all have a flexible, rod-shaped structure that supports the body called a notochord. Non-vertebrate chordates include tunicates, hagfish, and lancelets. In vertebrates the notochord ultimately becomes part of the spine, usually encased in bony joints. All chordates also have a dorsal hollow nerve cord that forms the nervous system and pharyngeal slits that open outside the body during development (and persist to form gills in aquatic animals). Lastly, chordates have a tail at the back of the body––it’s just that sometimes you need an x-ray to see it.

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

Introduction

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 proteins 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 Strategy

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 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.

simplified schematic diagram showing the structure of

Image: Marek M / Wikipedia / CC BY SA - Creative Commons Attribution + ShareAlike

In this schematic diagram, the yellow portions of each Y-shaped antibody are the variable binding sites. This portion of the antibody attaches to similarly-shaped structures, called epitopes, on the antigen shown at the center of the diagram.

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.

Last Updated April 19, 2018

References

“The biological function of antibodies is to bind to pathogens and their products, and to facilitate their removal from the body. An antibody generally recognizes only a small region on the surface of a large molecule such as a polysaccharide or protein. The structure recognized by an antibody is called an antigenic determinant or epitope. ” (Janeway et al. 2001:105)

“X-ray crystallographic analysis of antigen:antibody complexes has demonstrated that the hypervariable loops (complementarity-determining regions) of immunoglobulin V regions determine the specificity of antibodies. With protein antigens, the antibody molecule contacts the antigen over a broad area of its surface that is complementary to the surface recognized on the antigen. Electrostatic interactions, hydrogen bonds, van der Waals forces, and hydrophobic interactions can all contribute to binding. Amino acid side chains in most or all of the hypervariable loops make contact with antigen and determine both the specificity and the affinity of the interaction. Other parts of the V region play little part in the direct contact with the antigen but provide a stable structural framework for the hypervariable loops and help determine their position and conformation.” (Janeway et al. 2001:105)

Book

Immunobiology: The Immune System in Health and Disease

5th ed. New York (NY): Garland Publishing. 732 pp. | 21/06/2001 | Janeway CA Jr; Travers P; Walport M; Shlomchik M

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