Along with the double helix of DNA, the distinctive Y-shape of an antibody is one of the most recognized structures in biology and perhaps all of science. There are five classes of antibodies in humans and rodents defined by their respective immunoglobulin (Ig) heavy chains: IgG, IgM, IgD, IgA, and IgE. Here we focus on IgG, as it is one of the most abundant proteins in human serum (10-20% of total plasma protein), comprises 70-85% of the total immunoglobulin pool, has the longest plasma half-life (20-24 days), and it is the most common format used in antibody-based therapeutics. IgG (~150 kD) comprises four peptide chains: two identical heavy chains and two identical light chains connected by disulfide (S-S) bonds. The heavy chain in IgG comprise ~450 residues. There are four IgG subclasses in human (IgG1, IgG2, IgG3, and IgG4), five in mice (IgG1, IgG2a, IgG2b, IgG2c, IgG3), and four in rats (IgG1, IgG2a, IgG2b, IgG2c). The light chain in IgG comprise ~215 residues, and there are two light chain subclasses in humans, mice, and rat: kappa and lambda.
The two identical complementarity-determining regions (CDRs) at the upper tips of the Y-structure make up the antigen recognition surface for the antibody. There are 3 CDRs in the heavy chain and light chain (CDR1, CDR2, and CDR3), and in most cases antigen binding interactions require contributions from both heavy and light chain CDRs. Furthermore, in most cases the CDR3 region in the heavy chain is the key determinant of antigen recognition specificity. The CDRs are contained within the Fragment variable (Fv) domain (~25 kD): the variable heavy (VH) and variable light (VL) chain regions make up the Fv domain. The antigen-binding fragment (Fab, ~50 kD) domain includes the entire light chain (VL and constant light (CL)), the VH and the first constant domain of the heavy chain (CH1). Monomeric Fabs can be generated by digesting an antibody with papain protease. A divalent F(ab’)2 fragment (~100 kD) can be generated by digesting an antibody with Immunoglobulin-degrading enzyme from Streptococcus pyrogenes (IdeS). The fragment crystallizable (Fc, ~50 kD) region forms the base of the Y-structure and comprise the CH2 and CH3 domains of the heavy chain (HC). The Fc region binds to Fc receptors and complement proteins, and is often the site of modifications for therapeutic purposes (e.g. coupling to cytotoxic molecules), such is the case with antibody-drug conjugates (ADCs).