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 exposure to other substances that the body recognizes as harmful “non-self” antigens. This test measures the amount of immunoglobulins A, G, and M (IgA, IgG, IgM) in the blood and, under certain circumstances, in the cerebrospinal fluid (CSF) or saliva.
The first time a person is infected or exposed to a foreign substance (antigen), their immune system recognizes the microorganism or substance as “not their own” and stimulates plasma cells to produce specific immunoglobulins, also called antibodies, that can bind and neutralize the threat. With subsequent exposures, the immune system “remembers” the antigen found, allowing the rapid production of more antibodies and, in the case of microorganisms, helping to prevent reinfection.
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 the body’s first response to new infection or a new “non-self” antigen, providing short-term protection. They increase for several weeks and then decrease as IgG production begins.
Immunoglobulin G (IgG): About 70-80% of the immunoglobulins in the blood are IgG. Specific IgG antibodies are produced during an initial infection or exposure to another antigen, increasing a few weeks after it begins, then decreasing and stabilizing. The body maintains a catalog of IgG antibodies that can reproduce rapidly as long as it is exposed to the same antigen. IgG antibodies form the basis of long-term protection against microorganisms. In those with a normal immune system, enough IgG is produced to prevent reinfection. Vaccines use this process to prevent initial infections and add to the IgG antibody catalog by exposing a person to a weakened live organism or an antigen that stimulates recognition of the organism. IgG is the only immunoglobulin that can cross the placenta. The mother’s IgG antibodies protect the fetus during pregnancy and the baby during the first 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 it is also found in saliva, tears, respiratory and gastric secretions, and breast milk. IgA provides protection against infections in mucous areas of the body such as the respiratory tract (sinuses and lungs) and the gastrointestinal tract (stomach and intestines). When passed from mother to baby during breastfeeding, it helps protect the baby’s gastrointestinal tract. A baby does not produce significant amounts of IgA until after 6 months of age, so any IgA present in the baby’s blood before that date comes from breast milk. There are two subclasses of IgA: IgA1 and IgA2.
Immunoglobulin D (IgD): The role of IgD is not fully understood and IgD is not routinely measured.
Immunoglobulin E (IgE): IgE is associated with allergies, allergic diseases, and parasitic infections. It is almost always measured as part of a blood panel for allergy testing, but it is not generally included as part of a quantitative immunoglobulin test.
Immunoglobulin tests measure the total amount of each primary immunoglobulin class, IgA, IgM, and IgG, without distinguishing between subclasses. Separate tests can be performed to measure immunoglobulin subclasses and/or to detect and measure specific antibodies.
A variety of conditions can cause an increase (hypergammaglobulinemia) or a decrease (hypogammaglobulinemia) in immunoglobulin production. Some cause an excess or deficiency of all classes of immunoglobulins, while others affect only one class. Some of the conditions are passed down from one generation to the next (inherited) and some are acquired.