Immunoglobulin G （IgG） is a type of antibody. IgG is the most common type of antibody found in blood circulation. IgG molecules are created and released by plasma B cells. Each IgG antibody has two paratopes. Antibodies are major components of humoral immunity. IgG is the main type of antibody 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. In mice, the IgG subclasses are defined as IgG1, IgG2a/c, IgG2b, and IgG3. The IgG2c allele is expressed （instead of IgG2a） in certain inbred mouse strains, e.g., C57BL/6, C57BL/10, SJL, and NOD.