Immunoglobulin G (IgG) belongs to the immunoglobulin family and is a widely expressed serum antibody. It consists of a γ heavy chain in the constant (C) region. The monomeric 150kDa structure of IgG constitutes two identical heavy chains and two identical light chains with molecular weight of 50kDa and 25kDa, respectively. The primary structure of this antibody also contains disulfide bonds involved in linking the two heavy chains, linking the heavy and light chains and resides inside the chains. IgG is further subdivided into four classes namely, IgG1, IgG2, IgG3, and IgG4 with different heavy chains, named γ1, γ2, γ3, and γ4, respectively. Limited digestion using papain cleaves the antibody into three fragments, two of which are identical and contain the antigen-binding activity. The third fragment does not possess antigen-binding activity and is known as fragment crystallizable (Fc). It interacts with cells and effector molecules. The Fc fragment contains the CH2 and CH3 domains of the antibody molecule. Pepsin cleaves the carboxy-terminal side of the disulfide bonds in the general region of the antibody and this gives rise to the F(ab′)2 fragment. The two antigen-binding arms of the antibody are linked in this fragment. Maternal IgG is the only antibody transported across the placenta to the fetus. It passively immunizes the infants.
