Immunoglobulins play a key role in the body's immune system. They are proteins produced by specific immune cells called plasma cells in response to bacteria, viruses, and other microorganisms as well as exposures to other substances that are recognized by the body as "non-self" harmful antigens. This test measures the amount of immunoglobulins A, G, and M (IgA, IgG, IgM) in the blood and, in certain circumstances, in cerebrospinal fluid (CSF) or saliva.

The first time a person is infected or otherwise exposed to a foreign substance (antigen), their immune system recognizes the microorganism or substance as "non-self" and stimulates plasma cells to produce specific immunoglobulin(s), also called antibodies that can bind to and neutralize the threat. With subsequent exposures, the immune system "remembers" the antigen that was encountered, which allows for the rapid production of more antibodies and, in the case of microorganisms, helps prevent re-infection.

There are five classes of immunoglobulins and several subclasses. Each class represents a group of antibodies and has a slightly different role. Classes of immunoglobulins include:

Immunoglobulin M (IgM) – IgM antibodies are produced as a body's first response to a new infection or to a new "non-self" antigen, providing short-term protection. They increase for several weeks and then decline as IgG production begins.

Immunoglobulins G (IgG) – About 70-80% of the immunoglobulins in the blood are IgG. Specific IgG antibodies are produced during an initial infection or other antigen exposure, rising a few weeks after it begins, then decreasing and stabilizing. The body retains a catalog of IgG antibodies that can be rapidly reproduced whenever exposed to the same antigen. IgG antibodies form the basis of long-term protection against microorganisms. In those with a normal immune system, sufficient IgG is produced to prevent re-infection. Vaccinations use this process to prevent initial infections and add to the catalog of IgG antibodies, by exposing a person to a weakened, live microorganism or to an antigen that stimulates recognition of the microorganism. IgG is the only immunoglobulin that can pass through the placenta. The mother's IgG antibodies provide protection to the fetus during pregnancy and to the baby during its first few months of life. There are four subclasses of IgG: IgG1, IgG2, IgG3, and IgG4.

Immunoglobulin A (IgA) – IgA comprises about 15% of the total immunoglobulins in the blood but is also found in saliva, tears, respiratory and gastric secretions, and breast milk. IgA provides protection against infection in mucosal areas of the body such as the respiratory tract (sinus and lungs) and the gastrointestinal tract (stomach and intestines). When passed from mother to baby during breast-feeding, it helps protect the infant's gastrointestinal tract. Significant amounts of IgA are not produced by a baby until after 6 months of age so any IgA present in a baby's blood before then is from the mother's milk. There are two IgA subclasses: IgA1 and IgA2.

Immunoglobulin D (IgD) – the role of IgD is not completely understood and IgD is not routinely measured.

Immunoglobulin E (IgE) – IgE is associated with allergies, allergic diseases, and with parasitic infections. It is almost always measured as part of an allergy testing blood panel but typically is not included as part of a quantitative immunoglobulins test.

Immunoglobulin testing measures the total amount of each primary immunoglobulin class, IgA, IgM, and IgG, without distinguishing between subclasses. Separate testing can be performed to measure immunoglobulin subclasses and/or to detect and measure specific antibodies.