IgG (immunoglobulin G) accounts for 70% to 80% of the antibodies in the blood. They are produced during initial exposure to the antigen (often a bacteria or virus) and then increase within a few weeks before stabilizing. The body retains a catalogue of IgG antibodies that can be quickly produced in the event of exposure to the same antigen. IgG is therefore the basis for long-term immunity and vaccination. IgG levels are interpreted in light of IgA and IgM, the other two main classes of antibodies.
Low IgG levels can be due to genetic causes, but usually they are secondary to difficulties in the production of all types of immunoglobulins (immunosuppressive drugs, diabetes complications, kidney failure, etc.) or to protein loss through the kidneys, intestines or skin. Uncontrolled production of another class of immunoglobulin such as IgA or IgM (multiple myeloma) often causes a marked decline in the other classes.
High (polyclonal) levels of IgG are seen in acute and chronic infections, autoimmune diseases (rheumatoid arthritis, lupus), cirrhosis of the liver, etc. Isolated (monoclonal) increases in IgG can be of undetermined cause (MGUS) or associated with multiple myeloma, or chronic lymphocytic leukemia or lymphoma.