What Is IgG? - Definition, Deficiency, & Blood Test

Instructor: Aileen Staller
Globulins are a class of proteins that make up antibodies, which are part of the immune system and protect us from many diseases. Let's examine the most abundant globulin, called gamma globulin, its structure, function and what happens when it's deficient.

What Is Gamma Globulin IgG?

Gamma globulin, or immune globulin G (IgG) is the most abundant human antibody, accounting for 75% of human antibodies. IgG is a short chain of amino acids connected by peptide bonds, and each molecule has 2 antigen binding, which it uses to protect use from invading organisms. Antigens are the diseases or organisms (like fungi, yeast, bacteria, and viruses) that cause an immune response in the body when the invader comes in contact with the immune globulins. Antibodies are the protectors, which match up with antigens and attempt to kill or disable them. The ability of the body to produce enough antibodies specific to the invader in a short enough time determines the strength of the immune response.

Action of Antibodies

Antibodies are produced by cells in the immune system called plasma B cells, and are found in all body fluids. They attach to certain, specific antigens, much like a lock and key system. When a person is exposed to antigen A, antibodies are produced that are mirror images of the binding site on antigen A. These antibodies will then bind to antigen A, but will not bind to antigen D or XX or 732.1 (if such antigens existed). The binding removes the antigen from circulation (this is called agglutination) and prevents it from further activation of the immune system. The binding also produces a clumping of antigen and antibody that further immobilizes the antigen. And this binding allows the antibody to produce a coating (opsonization) over the antigen, which allows the body to recognize and digest the invading antigen.

These antibody responses also activate a classical physiologic pathway (known as complement production), which produces additional protein capable of destroying the invader, neutralizing toxins it may produce, and_allowing destruction of the proteins in the invader, resulting in death of the organism. Antibodies typically appear 24-48 hours after exposure to the antigen and there are other forms of antibodies in addition to IgG, although it is the most abundant.

Measurement of IgG

IgG can be assessed and measured by laboratory tests, which measure the specific proteins for each type and sub type of antibody. For example, testing to determine the response to vaccination for mumps, measles, rubella, hepatitis B and chickenpox involves testing for IgG levels. However, IgG is not an antibody involved in allergic reactions.

IgG Structure

IgG is a small molecule and is able to cross the placenta, which confers immunity to the fetus. IgG is also found in colostrum (early breast milk) and helps confer immunity to the newborn. Under normal immune conditions, once a body is exposed to an antigen it will recognize that antigen when exposed again and restart antibody production.

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