What is the structure of immunoglobulin G (IgG)?

An IgG antibody is a large monomeric molecule with a tetrameric quaternary structure and a molecular weight of about 150 kDa. It is composed of 4 peptide chains - 2 identical gamma heavy chains (γ heavy chains ) weighing about 50 kDa each and 2 identical light chains, which may be either Kappa (κ) or Lambda (λ), weighing about 25 kDa each.

Disulfide bonds interconnect the heavy chains to each other and to a light chain, forming a Y-shaped tetramer with identical halves. Each end of the Y-shaped antibody has an identical antigen-binding site, resulting in two antigen-binding sites per IgG antibody.

An IgG antibody can be divided into two fragments - the antigen-binding Fab fragment and the Fc or crystallizable fragment. 

The Fab fragment contains 4 domains: VL (Variable region of Light chain), VH (Variable region of Heavy chain), CL (Constant region of Light chain), and CH1 (Constant region of Heavy chain). It also has a variable antigen-binding site known as the Fv region, which differs from one antibody to another.

The Fc region, forming the trunk of the Y-shaped antibody, is composed of the constant domains of the heavy chains and bears a highly conserved N-glycosylation site.

This highly specialized structure enables IgG antibodies to effectively recognize and bind antigens, playing a crucial role in the immune response.