IgG (H+L): Antibodies

Antibodies, also known as immunoglobulins (Igs) are critical for immunity and are grouped into five primary classes: IgG, IgM, IgA, IgD, and IgE. The most abundant antibody isotype is immunoglobulin G (IgG) with concentrations ranging from 7.5-22 mg/ml in human serum and has a molecular weight of 150 kDa. The major effector functions of IgG include neutralization, opsonization, complement fixation and antibody dependent cell-mediated cytotoxicity (ADCC). This monomeric immunoglobulin, expressed on the surface of mature B cells, is often depicted as a Y-shape and comprised of 2 heavy chains and 2 light chains linked by disulfide bonds. The heavy chain is type gamma including subtypes gamma 1, gamma 2, gamma 3, and gamma 4 while the light chain is either a kappa or lambda chain. An IgG molecule has two antigen binding sites, each consisting of a heavy and light chain N-terminal variable domain. When combined with the constant heavy chain 1 (Ch1) and the constant light chain domains, it forms the fragment antigen-binding (Fab) region (2 per antibody). The remaining domains (Ch2-Ch4) of both heavy chains make up the Fc region and contain a site for covalently linking an enzymatic or fluorochrome probe, such as HRP or Janelia Fluor 549, for target detection and visualization (1,2,3).

The 4 IgG subclasses, sharing 95% amino acid identity, include IgG1, IgG2, IgG3, and IgG4 for humans and IgG1, IgG2a, IgG2b, and IgG3 for mice. The relative abundance of each human subclass is 60% for IgG1, 32% for IgG2, 4% for IgG3, and 4% for IgG4. In an IgG deficiency, there may be a shortage of one or more subclasses (4).

References

1. Painter RH. (1998) Encyclopedia of Immunology (Second Edition). Elsevier. 1208-1211

2. Chapter 9 - Antibodies. (2012) Immunology for Pharmacy. Mosby 70-78

3. Schroeder H, Cavacini, L. (2010) Structure and Function of Immunoglobulins. J Allergy Clin Immunol. 125(2 0 2): S41-S52. PMID: 20176268

4. Vidarsson G, Dekkers G, Rispens T. (2014) IgG subclasses and allotypes: from structure to effector functions. Front Immunol. 5:520. PMID: 25368619