An illustration of IgG1, a subclass of Immunoglobulin G and the most abundant antibody produced by plasma B cells that circulate in the body to defend against infection. After binding to a foreign substance, antibodies can overwhelm and immobilize pathogens, or act as markers to signal other cells in the immune system. The inset in the illustration depicts a ribbon model of the antibody, a quaternary structure composed of two heavy and light peptide chains, which link to form two antigen binding sites.