Immunoglobulin Function and Structure

IgG

IgG is the class of immunoglobulin that is most abundant. It is found in the plasma and is characterized by gamma heavy chains. Moreover, IgG is a monomeric immunoglobulin siotype, divided in humans into subclasses IgG 1,2,3 and 4. IgG is small, and can easily diffuse out of the blood into the tissues and is also the principle isotype found in the blood and extracellular fluid. In addition, IgG efficiently opsonized pathogens for engulfment and by phagocytes and activates complement system, as IgG operates in the body tissues where accessory cells and molecules are available. In addition, IgG is transported across the placenta directly into the bloodstream of the fetus during gestation, providing the necessary protection for a fetus. (1)

Structure and Function

The functions of IgG are dictated by its' versatile small structure. IgG antibodies consist of four polypeptide chains, which forms three equal-sized globular portions joined by a flexible hinge region, compromised by a stretch of polypeptide, forming a flexible 'Y' shape. IgG has a molecular weight of 150 kDa and is composed of two different polypeptide chains. The heavy chain (H) is about 50 kDa, while the other, the light chain (L), is 25 kDa. The two chains are gound at equimolar rations, and each IgG molecule contains two heavy chains and two light chains. The two heavy chains are linked with a pair of disulfide bonds and each heavy chain is linked to a light chain also by disulfide bonds, but only by a single bond. Both the heavy and light chains of the IgG can be divided into domains on the basis of sequence similarity. The two domains of the light chains are VL and CL. IgG has four domains in its heavy chain, which are termed VH, CH1, CH2, and CH3. These domains are about 110 amino acids in length, with the amino-terminal end of both the heavy and light chains varying greatly. These variable domains (V) make up the V region of the antibody and determine its specificity, while the constant domains determine the effector mechanisms of an antibody once interaction with antigen is achieved, as well as its immunoglobulin isotype class IgG. (1,4)

References:

1. Janeway, C.A., Travers, P., Walport, M., and Capra, J.D. 1999. Immunobiology: The immune system in health and disease. Garland Publishing. 4th ed., New York, USA

2. Delves, P. and Roitt, I. 1999. Encyclopedia of Immunology: Academic Press Inc., 2nd ed., San Diego, USA

3. 1994. Current Protocols in Molecular Biology. Volume 2. John Wiley & Sons Inc., USA

4. Cruse, J. and Lewis, R. 1995. Illustrated Dictionary of Immunology. CRC Press Inc., USA