IgG (Immunoglobulin G) is the main type of antibody found in blood and extracellular fluid. It protects the body from infection by binding several types of pathogens including bacteria, viruses, and fungi.

IgG antibodies are used for the laboratory diagnosis of viral infections to determine immune status and to diagnose persistent infection. IgG antibody responses are considered by some to be a useful marker of antigen exposure, especially in allergic individuals receiving allergen immunotherapy or individuals receiving gamma globulins or protein and polysaccharide vaccines to diagnose immunodeficiency.