Pathology Laboratory

What is an Antibody?

An antibody is a protein produced by the immune system. Antibodies help fight infections. Antibodies are created by the immune system as a defense.

Antibodies

Antibodies are cells in the immune system responsible for recognizing and combating sickness. They are created by the body when it is infected by a foreign entity. They can also be found in breast milk, colostrum, and other fluids in order to protect infants against bacteria and viruses that could harm them. Antibodies work by binding to an invader, which then allows the immune system to remove it.

Antibodies: The Crucial Cells that Fight Against Disease

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

What is an antibody?

An antibody is a protein produced by the immune system that can bound with high specificity to an antigen. These antigens are typically other proteins but maybe carbohydrates, small molecules, or even nucleotides. Antibodies are powerful research tools because they bind specifically to a unique epitope on the antigen, allowing the detection of a specific protein in an assay while avoiding the detection of unrelated proteins. This specific binding capability also allows antibodies to be used in diagnostic applications, such as pregnancy tests and in therapeutic applications, such as cancer treatments.

The antibody itself is a Y-shaped protein that contains a constant region common to all antibodies produced by a particular species and a variable region that is unique and specific to a particular epitope. 

Antibodies that bind specifically to the antigen of interest can be used in several immunoassays. For example, Western Blot and ELISA assays allow for detection and quantification of specific proteins. 

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 structures 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 primary 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. Sometimes, the interaction of the B cell with a T helper cell is also necessary.

Antibodies and Antigens in Blood

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 a solution or in 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

A 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 is the 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, several 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.

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This study was originally published by Dr. Ananta Mandal, MD in News Medical Life Sciences. Image: Source.

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