Immunoglobulin G (IgG) is a type of antibody created and released by plasma B cells. IgG is the main type of antibody in humoral immunity found in blood and extracellular fluid allowing it to control infection of body tissues. By binding many kinds of pathogens such as viruses, bacteria, and fungi, IgG protects the body from infection. There are four IgG subclasses (IgG1, 2, 3, and 4) in humans, and four IgG subclasses (IgG1, 2a, 2b, 3) in mice and rats. The measurement of IgG can be a diagnostic tool for certain conditions, such as autoimmune hepatitis, if indicated by certain symptoms. Clinically, measured IgG antibody levels are generally considered to be indicative of an individual’s immune status to particular pathogens.