IgG

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Immunoglobulin G (IgG) is an antibody, which is abundant in the blood, lymph and extracellular fluid [1].

IgG plays various roles in the immune response, but primarily, it aids the phagocytosis of pathogens and alleviates their destruction [2]. IgG acts by coating the microorganism, a process known as 'opsonization' (UK spelling: opsonisation) and then binds to phagocytes through its constant regions (Fc receptors). Being coated by the IgG, the pathogen is captured by the neutrophils and the macrophages and finally undergoes destruction by the latter [3].

During pregnancy, IgG are the only type of human antibodies that can pass from mother to the foetus. Cells at the placenta have Fc receptors that bind to the IgG antibodies in the mother’s blood. Consequently, IgG is endocytosed in the form of a vesicle across the cell and expelled into the fetus’ blood[4].

Structure

IgG has a molecular weight about 150kDa. It is composed of two light chains and two heavy gamma chains, each gamma chain having 4 domains. There are 4 subclasses of IgG; IgG1 (γ1 heavy chains), IgG2( γ2 heavy chains), IgG3 (γ3 heavy chains) and IgG4 (γ4 heavy chains). Furthermore isotypes exist with the light chains either being kappa κ or lambda λ[5]. These subclasses give rise to variations in properties and abundance of each type.

At the end the N terminus of each chain there is a variable region, known as the fragment antigen binding site Fab. Three hypervariable loops from each chain are the determining region and are highly specific to the complimentary antigen. Altogether six hypervariable regions (3 from the light chain and 3 from the heavy chain) form a Fab and each IgG antibody has 2 fragment antigen binding sites. The constant region, known as the Fragment Crystallisable Fc, interacts with receptors on cells during secondary immune responses and binds to C1q in the Classical Complement Pathway.
The IgG antibody chains are configured to produce a Y-shaped unit. The two heavy chains are joined by a hinge region, an intra-chain disulphide bond that allows the flexibility and movement of the Fab domains. The light variable chain and the heavy variable chain are also joined by an intra-chain disulphide bond[6].

References:

  1. Parham,P. (2009) The Imuune System, 3rd edition, New York, Garland Science
  2. Parham, P. (2009)The Immune System, 3rd edition, New York, Garland Science
  3. Parham, P. (2009)The Immune System, 3rd edition, New York, Garland Science
  4. Janeway CA Jr, Travers P, Walport M, et al (2001) Immunobiology: The Immune System in Health and Disease, 5th edition, New York: Garland Science; 2001.
  5. Alberts B, et al (2008) Molecular Biology of the Cell, 5th edition, New York: Garland Science; p1554-5
  6. Janeway CA Jr, Travers P, Walport M, et al (2001) Immunobiology: The Immune System in Health and Disease, 5th edition, New York: Garland Science; 2001.
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