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Immunoglobulin G Antibody
Immunoglobulin G is the most abundant immunoglobulin and is found in all body fluids. Each Y-shaped molecule has two arms (top) that can bind to specific antigens. In doing this they mark the antigen for destruction by phagocytes, white blood cells that ingest and destroy foreign bodies. Antibodies can also kill some pathogens directly, and can neutralize toxins. ALFRED PASIEKA/Science Photo Library/Getty Images

What Are Antibodies?

Antibodies (also called immunoglobulins) are specialized proteins that travel thorough the blood stream and are found in bodily fluids. They are utilized by the immune system to identify and defend against foreign intruders to the body. These foreign intruders, or antigens, include any substance or organism that evokes an immune response. Bacteria, viruses, pollen, and incompatible blood cell types are examples of antigens that cause immune responses. Antibodies recognize specific antigens by identifying certain areas on the surface of the antigen known as antigenic determinants. Once the specific antigenic determinant is recognized, the antibody will bind to the determinant. The antigen is tagged as an intruder and labeled for destruction by other immune cells. Antibodies protect against substances prior to cell infection.

Antibody Production

Antibodies are produced by a type of white blood cell called a B cell (B lymphocyte). B cells develop from stem cells in bone marrow. When B cells become activated due to the presence of a particular antigen, they develop into cells called plasma cells. Plasma cells create antibodies that are specific to a specific antigen. Plasma cells generate the antibodies that are essential to the branch of the immune system known as the humoral immune system. Humoral immunity relies on the circulation of antibodies in bodily fluids and blood serum to identify and counteract antigens.

When an unfamiliar antigen is detected in the body, it can take up to two weeks before plasma cells can generate enough antibodies to counteract the specific antigen. Once the infection is under control, antibody production decreases and a small sample of antibodies remain in circulation. If this particular antigen should appear again, the antibody response will be much quicker and more forceful.

Antibody Structure

An antibody or immunoglobulin (Ig) is a Y-shaped molecule. It consists of two short polypeptide chains called light chains and two longer polypeptide chains called heavy chains. The two light chains are identical to each other and the two heavy chains are identical to each other. At the ends of both the heavy and light chains, in the areas that form the arms of the Y-shaped structure, are regions known as antigen-binding sites. The antigen-binding site is the area of the antibody that recognizes the specific antigenic determinant and binds to the antigen. Since different antibodies recognize different antigens, antigen-binding sites are different for different antibodies. This area of the molecule is known as the variable region. The stem of the Y-shaped molecule is formed by the longer region of the heavy chains. This region is called the constant region.

Antibody Classes

Five primary classes of antibodies exist with each class playing a distinct role in the human immune response. These classes are identified as IgG, IgM, IgA, IgD and IgE. Immunoglobulin classes differ in the structure of the heavy chains in each molecule.

Immunoglobulins (Ig)

  • IgG: These molecules are the most plentiful in circulation. They can cross blood vessels and even the placenta to provide protection to a fetus. The heavy chain type in IgG is a gamma chain.
  • IgM: Of all of the immunoglobulins, these are the most massive. They contain five Y-shaped sections each with two light chains and two heavy chains. Each Y-shaped section is attached to a joining unit called a J chain. IgM molecules play a major role in the primary immune response as the initial respondents to new antigens in the body. The heavy chain type in IgM is a mu chain.
  • IgA: Located mainly in body fluids such as sweat, saliva, and mucus, these antibodies prevent antigens from infecting cells and entering the circulatory system. The heavy chain type in IgA is an alpha chain.
  • IgD: The role of these antibodies in the immune response is currently unknown. IgD molecules are located on the surface membranes of mature B cells. The heavy chain type in IgD is a delta chain.
  • IgE: Found mostly in saliva and mucus, these antibodies are involved in allergic responses to antigens. The heavy chain type in IgE is an epsilon chain.

There are also a few subclasses of immunoglobulins in humans. The differences in subclasses are based on small variations in the heavy chain units of antibodies in the same class. The light chains found in immunoglobulins exist in two major forms. These light chain types are identified as kappa and lambda chains.

National Human Genome Research Institute
National Institute of Allergy and Infectious Diseases