Immunoglobulins are proteins which play an essential role in the body's defences against assault.
They belong to the group of gamma globulins which are not only found in the blood (specifically in serum: the liquid part of blood, which is the plasma with some coagulation agents removed) but also in other body fluids.
The most abundant are IgG (75 to 80% of circulating antibodies). They are produced following contact with an antigen (a foreign body). They protect the body against bacteria, viruses, and toxins which circulate in blood and lymph. They also bind rapidly to the complement (one of the component parts of the immune system). They also contribute to the immune memory response that forms the basis of the vaccination mechanism. Finally, immunoglobulin G crosses the placenta and provides passive immunity to the fetus.
IgA is mainly found in secretions such as saliva, intestinal juices, sweat and breast milk. The essential role of immunoglobulin A is to prevent pathogens from binding to the cell, particularly to lining cells which form the mucosal membranes and epidermis (the superficial layer of the skin).
The IgM are immunoglobulins secreted following initial contact between the body and an antigen. This is the first class of immunoglobulins to be released by a range of white blood cells, the plasmocytes. The presence of IgM in the blood indicates current infection.
IgD is a variety of immunoglobulin which is usually attached to the surface of B lymphocytes where it acts as a receptor for antigens. It is believed to be involved in maturation of lymphocytes, i.e. the mechanism activating these white cells.
IgE are larger then immunoglobulin G and are secreted by a variety of white globules, the plasmocytes, in skin, the gastro-intestinal tract, the tonsils, and the respiratory tract. This type of immunoglobulin is bound to mastocytes and basophilic granulocytes (types of white blood cell) by a stem. As soon as an antigen is captured, the immunoglobulin triggers the release of the substance that contributes to the inflammatory reaction, and histamine that is involved in the allergic reaction. Immunoglobulin concentrations rise in severe allergy, and when parasites are present in the gastric-intestinal tract (parasitoses).