IgG is a monomeric immunoglobulin composed of two heavy chains and two
light chains. There are four subclasses of the IgG: IgG1, IgG2, IgG3 and IgG4.
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.