An immunoglobulin (Ig), also known as an antibody (Ab), is a Y-shaped protein, produced by B-cells and plasma cells, that is used by the immune system to identify and neutralize foreign antigens such as bacteria, viruses, and parasites. Immunoglobulins come in different varieties known as isotypes or classes. In placental mammals, there are five antibody isotypes known as:
- and IgM
They are each named with an “Ig” prefix that stands for immunoglobulin and differ in their biological properties, functional locations, and ability to deal with different antigens:
Immunoglobulin A: Protects against infections of the mucous membranes lining the mouth, airways, and digestive tract. Found in mucosal areas, such as the gut, respiratory tract and urogenital tract, and prevents colonization by pathogens. Also found in saliva, tears, and breast milk.
Immunoglobulin D: Remains in the bloodstream to fight bacteria. Functions mainly as an antigen receptor on B cells that have not been exposed to antigens. It has been shown to activate basophils and mast cells to produce antimicrobial factors.
Immunoglobulin E: Binds to allergens and triggers histamine release from mast cells and basophils; is involved in allergic reactions. Also protects against parasitic worms. Frequently increased in parasitic infestations and atopic individuals (with allergic hypersensitivity).
Immunoglobulin G: Major type of antibody found in the blood that can enter tissues and fight infection. In its four forms, provides the majority of antibody-based immunity against invading pathogens. The only antibody capable of crossing the placenta to give passive immunity to the fetus.
Immunoglobulin M: Expressed on the surface of B cells (monomer) and in a secreted form (pentamer) with very high avidity (forms multiple binding sites with antigen). Eliminates pathogens in the early stages of B-cell mediated (humoral) immunity before there is sufficient IgG. Found mainly in the blood and lymph fluid, and is the first to be made by the body to fight a new infection.
Find out what low and high numbers mean and what the reference ranges for each of the above markers are @