Immunoglobulin

Immunoglobulins are proteins produced by, and active in, the immune system. They are produced by B-lymphocytes, from the effects of both genes and antigens. Immunoglobulins are responsible for humoral immunity, or immune reactions controlled by antibody molecules in the body.

Structurally, they have two heavy and two light chains, and, depending on the specific immunoglobin, additional polypeptide chains. They can stay as single proteins, or polymerize into more complex structures.

The transmembrane forms become antigen receptors on the B-cells. When activated, the B-cells secrete immunoglobins as antibodies.

They are divided into major classes based on the amino acid sequencing of their heavy chains. The classic way to separate the major classes is electrophoresis, although much more complex methods are used to recognize specific antibodies. All immunoglobulins are part of the gamma (γ) globulin class, so they are abbreviated either IgX or γX, where X is a class type. The classes can be separated by a second electrophoresis.

Immunoglobulin A

The second most common of the immunoglobulins, IgA is produced by lymphoid tissue lining the gastrointestinal, genitourinary, and respiratory tracts. When secreted, they combine with another protein, secretory component, which help its transport into secretions and protect it from proteolytic enzymes in those secretions.

IgA receptors are on lymphocytes, polymorphonuclear neutrophils and monocytes.

Immunoglobulin D

Of these proteins, the role of IgD is least understood. It appears to be associated with the creation of additional circulating B-lymphocytes, and as a surface receptor on lymphocytes rather than as a circulating antibody produced by lymphocytes. If so, it is acting as a cytokine rather than an antibody.

Immunoglobulin E

IgE is the key to the inflammatory response. Much like IgA, it is associated with secretions rather than circulating in blood; it occurs in the respiratory and intestinal tracts.

Immunoglobulin G

This is the most common immunoglobin protein, with a long half-life. It is present both inside and outside of blood vessels, and is likely to have a role against blood-borne pathogens. It activates complement.

Receptors for IgG are on some lymphocytes, monocytes, polymorphonuclear neutrophils, and tissue macrophages of the spleen and liver. The latter are sometimes called part of the reticuloendothelial system.

Immunoglobulin M

IgM molecules are, by far, the largest of the molecules, which keeps them inside blood vessels. They clump together cells bearing antigens (e.g., bacteria) in a process called agglutination. Peak IgM production takes place in the initial response to an antigen, before the peak IgG production; IgM production drops off faster than does IgG.