Immunoglobulin G （IgG） is a type of antibody. 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. IgG antibodies are large globular proteins made of four peptide chains; two identical γ （gamma） heavy chains of about 50 kDa and two identical light chains of about 25 kDa. light chain （L chain） refers to the small molecular weight peptide chain in immunoglobulin. According to its structure and the antigenicity of the constant region, it is divided into two types: kappa light chain and lambda light chain.