Test Details: In the body, majority of the immunoglobulins in the blood belong to the IgG class of immunoglobulins. When there is infection caused by any specific antigen, IgG antibodies are produced later in the course of infection. The body retains a catalogue of these IgG antibodies that can be rapidly reproduced whenever exposed to the same antigen anytime in the future. Thus, 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. IgG is the only immunoglobulin that can pass through the placenta. The mothers 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.