IgG is a monomeric immunoglobulin composed of two heavy chains and two light chains. There are 4 subclasses of the IgG: IgG-1, IgG-2, IgG-3 and IgG-4. Each molecule has two antigen binding sites. IgG is the most abundant immunoglobulin as well as the only isotype that can pass through the placenta, thereby providing protection to the fetus in its first weeks of life before its own immune system has developed. IgG can bind to several different kinds of pathogens, for example viruses, bacteria, and fungi, and it protects the body against them by complement activation (the classic pathway), opsonization for phagocytosis, and neutralization of their toxins.
Ig gamma-1 chain C region; IGHG1