Antibody - What is an Antibody?

By Dr Ananya Mandal, MD

Antibodies are large Y-shaped proteins. They are recruited by the immune system to identify and neutralize foreign objects like bacteria and viruses.

Humoral immune system

Each antibody has a unique target known as the antigen present on the invading organism. This antigen is like a key that helps the antibody in identifying the organism. This is because both the antibody and the antigen have similar structure at the tips of their “Y” structures.
Just like every lock has a single key, an antibody has a single antigen key. When the key is inserted into the lock, the antibody activates, tagging or neutralizing its target. The production of antibodies is the main function of the humoral immune system.

Antibodies and immunoglobulins

Immunoglobulins are basically proteins that function as antibodies. The terms antibody and immunoglobulin are often used interchangeably.

Immunoglobulins are found in blood and other tissues and fluids. They are made by the plasma cells that are derived from the B cells of the immune system. B cells of the immune system become plasma cells when activated by the binding of a specific antigen on its antibody surfaces. In some cases, the interaction of the B cell with a T helper cell is also necessary.

Antibodies and antigens

Antigens are classically defined as any foreign substance that elicits an immune response. They are also called immunogens. The specific region on an antigen that an antibody recognizes and binds to is called the epitope, or antigenic determinant.

An epitope is usually made up of a 5-8 amino acid long chain on the surface of the protein. The chain of amino acids does not exist in a 2 dimensional structure but appears as a 3 dimensional structure. An epitope may only be recognized in its form as it exists in solution, or its native 3D form. If the epitope exists on a single polypeptide chain, it is a continuous, or linear epitope. The antibody may bind to only fragments or denatured segments of a protein or to the native basic protein.

Types of antibodies and their structures

Serum containing antigen-specific antibodies is called antiserum. There are five classes of immunoglobulins including IgM, IgG, IgA, IgD, and IgE.

The basic structure of all antibodies are same. There are four polypeptide chains held together by disulfide bonds. These four polypeptide chains form a symmetrical molecular structure.

There are two identical halves with the antigen binding sites between the ends of the heavy and light chains on both sides. There is a hinge in the center between heavy chains to allow flexibility to the protein. The two light chains are identical to each other. They contain around 220 amino acids while the heave chains have 440 amino acids.

There are two types of light chain among all classes of immunoglobulin, a lambda chain and a kappa chain. Both are similar in function. Each type of immunoglobulin has a different type of heavy chain.

Antibody functions

The antibody binds to specific antigens. This signals the other cells of the immune system to get rid of the invading microbes. The strength of binding between the antibody and an antigen at a single binding site is known as the antibody’s affinity for the antigen. The affinity between the antibody and the antigen binding site is determined by the type of bond formed.

Since an antigen can have multiple different epitopes, a number of antibodies can bind to the protein. When two or more antigen binding sites are identical, an antibody can form a stronger bond with the antigen.

Reviewed by April Cashin-Garbutt, BA Hons (Cantab)

Further Reading

Last Updated: Jun 4, 2013

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