IgG : Structure, Property and Machanism of Action

  Immunoglobulins or antibodies are divided into five main classes. These are IgG, IgA, IgM, IgD and IgE. The basis of this classification is depends on the constant region of heavy chains. these regions are unique to each class. Each of these classes have distinct physical properties and biological effector functions.

Immunoglobulin G (Ig G)

Structure of Ig G

• Ig G antibody is made up of two gamma(γ) heavy chains and two light chains. 

• These chains are held together by disulfide bonds.

• Each heavy chain has four domains, one variable domain at the end terminal and three constant domains. so immunoglobulin G is a monomer.

Structure of IgG

Concentration of Ig G

• IgG is the most abundant antibody in the serum. It comprises of about 80 to 85% of the antibodies in the serum.

• This antibody is most commonly found in circulation and tissues.  Besides blood these are predominantly found in lymph fluid, cerebral spinal fluid and peritoneal fluid.

Half life of Ig G

• It's half-life in serum is 23 days. by half-life mean the time required for half of the antibodies to disappear. This is the longest half life among antibody classes and therefore they are predominant in the serum.

• Because of its long half-life immunoglobulin G provides long term immunity.

Subclasses of Ig G

• Ig G class of antibodies in humans is divided into four sub classes namely : IgG1, IgG2, IgG3 and IgG4. They are numbered in order to abundance in a serum.

• Among these IgG1 is the most abundant subclass.
IgG1 > IgG2 > IgG3 > IgG4

• These subclasses are 90 to 95% similar to each other. They vary in their heavy chain composition and the arrangement and number of disulfide bonds.

IgG 3 has maximum number of disulfide bonds in the hinge region. In fact the hinge region is IgG 3 is the longest.

important properties and functions of IgG antibodies

• IgG is the only class of immunoglobulin that can pass through a placenta. This property of IgG enables mother to transfer her immunity to the fetus. This is very important because immune system of a newborn is not fully developed. IgG antibodies from mother confers passive immunity to a fetus.

• Among the subclasses of IgG except IgG 2 all other are able to cross the placenta and enter the fetal circulation.

• IgG antibodies are able to enter tissue spaces. This is important when there are any antigens present in tissue spaces.
- IgG antibodies code them and speed up their uptake by phagocytic cells.
- IgG is an effective optimizing antibody. IgG 1 and IgG 3 has high affinity to FC receptors or phagocytic cells. Thus they increase the efficiency of phagocytosis.

• It is also effective in complement activation. These antibodies activate complement by classical pathway.

• It also contributes an antibody dependent cell-mediated cytotoxicity.
- Lytic cells such as natural killer cells have FC gamma receptors on their surface that bind FC region of IgG antibodies.

• Ig G antibodies neutralize toxins and viruses it is an excellent neutralizing antibody.