Immunoglobulin G (IgG) is one of the most abundant proteins in serum. IgG is important for our defense against microorganisms. IgG molecules are produced by B lymphocytes, which are part of our adaptive immune response. IgG molecules have two independent functions: binding to pathogens that cause reactions and recruiting other cells and molecules to destroy antigens. Immunoglobulin G (IgG) is the main immunoglobulin component in serum, accounting for about 75% of the total immunoglobulin content in serum, with normal levels in adult blood ranging from 8-17 mg/ml. There are 4 subclasses: IgG1 (60~70%), IgG2 (15~20%), IgG3 (5~10%) and IgG4 (1~7%).