Antibodies are proteins made by the immune system to destroy foreign invaders. There are five major types of antibodies: immunoglobulin A (IgA), immunoglobulin G (IgG), immunoglobulin M (IgM), immunoglobulin E (IgE) and immunglobulin D (IgD). An IgG antibody helps to battle bacterial and viral infections. Comprising up to 80% of the all of these essential proteins, it is the smallest, yet most abundant, human antibody. Found in all bodily fluids, IgG is the only antibody that can protect a fetus by passing through the mother's placenta.
A physician may order a quantitative immunoglobulin blood test to determine the exact level of each type of antibody if the results of either a total blood protein test or serum protein electrophoresis test — which can measure each type of protein in the blood — are abnormal. The doctor may also order a quantitative immunoglobulin test if the patient's symptoms suggest an autoimmune disease, allergies, certain blood cancers, or recurring infections. The test is also used to follow-up on the treatment for Helicobacter pylori (H. pylori) bacteria and to confirm a response to a vaccination already given, to ensure that the patient has achieved immunity to the disease.
Normal levels of the IgG antibody usually fall in the range of 565–1765 mg/dL (5.65–17.65 g/L). A higher than normal level can suggest an IgG monoclonal gammopathy, such as multiple myeloma — a cancer of the blood and bone marrow — or monoclonal gammopathy of undetermined significance (MGUS) — a benign, but sometimes pre-malignant condition. Elevated IgG antibody levels may also suggest the presence of a chronic infection with an illness such as AIDS or hepatitis. Multiple sclerosis (MS), a chronic neurological disease of the central nervous system, is another possible diagnosis when the IgG antibody level is higher than normal.
A lower than normal IgG antibody level may suggest some types of leukemia or nephrotic syndrome, which often results in kidney damage. Low levels of the IgG antibody can also be caused by Waldenstrom's macroglobulinemia, a rare blood cancer that is characterized by an overproduction of the IgM antibody and subsequent suppression of production of other types of antibodies, including IgG. In addition, some primary immunodeficiencies are defined by a low level or complete deficiency of IgG. These include: X-Linked Agammaglobulinemia (XLA), a congenital disease in which IgG, IgM and IgA antibody levels are all greatly reduced or non-existent; Common Variable Immunodeficiency (CVID), also known as hypogammaglobulinemia), a condition in which there is a reduction in the number of one or more of the main three antibodies (IgG, IgM or IgA); and Severe Combined Immunodeficiency, a genetic illness that usually presents with a reduction in all classes of antibodies.
Primary immunodeficiencies may be treated with Intraveneous Immunoglobulin (IVIG) therapy, which contains purified IgG antibodies collected from healthy donors. IVIG helps to protect immunodeficient patients by temporarily replacing the antibodies needed to fight infections. IVIG therapy is also used to treat some auto-immune and neurological diseases, such as dermatomyositis/polymyositis, idiopathic thrombocytopenic purpura(ITP), Guillain-Barre syndrome, and chronic inflammatory demyelinating polyneuropathy (CIDP).