A monomeric isotype of antibody or immunoglobulin characterized by its γ-heavy chain, and in humans, is the most predominant in the serum and the only immunoglobulin capable of passing across the placenta to the fetus
The plasma cell produces immunoglobulins (or antibodies) as an immune response to an antigen, i.e. non-self or foreign agent. An immunoglobulin (Ig) is a glycoprotein with a Y-shape. The basic structure of a monomeric unit of antibody consists of two identical heavy chains and two identical light chains. In each of these chains, there are variable and constant regions. The immunoglobulin heavy chain, in particular, is the large polypeptide subunit. It can be used to classify the various isotypes (or classes) of immunoglobulins. In humans and other mammals, the five isotypes are: (1) immunoglobulin A (IgA), (2) immunoglobulin D (IgD), (3) immunoglobulin E (IgE), (4) immunoglobulin G (IgG), and (5) immunoglobulin M (IgM).
The immunoglobulin G (IgG) has a gamma-type of immunoglobulin heavy chain (thus, the acronym G). Its light chain can either be a kappa or a lambda. Thus, its molecular formula: γ2 κ2 or γ2 λ2. IgG is a monomer. It means it has only one functional unit of antibody. The molecular weight is 150 kDa (except for IgG3, 170 kDa). In humans, four main subclasses are known (i.e. IgG1, IgG2, IgG3, and IgG4). IgG2 differs from the rest in not being transferred across the placenta and IgG4 does not fix complement.
IgG is the most common type of antibody in the blood. It accounts for about 75% of the serum antibodies in humans. IgG is present at 8-16 mg/ml in serum.
IgGs act on pathogens by agglutinating them, by opsonising them, by activating complement-mediated reactions against cellular pathogens, and by neutralising toxins. They can pass across the placenta to the foetus as maternal antibodies, unlike other Ig classes. Thus, it can be passed on from mother to foetus, thereby giving passive immunity to the foetus.