Immunoglobulin G: A class of immunoglobulins found in all body fluids. They are the smallest but most common antibodies (75 percent to 80 percent) in the body. Abbreviated IgG. IgG antibodies are very important in fighting bacterial and viral infections. IgG antibodies are the only type of antibody that can cross the placenta in a pregnant woman to help the fetus.
Last Editorial Review: 5/13/2016