Immunoglobulin G

Immunoglobulin G (IgG) is a type of antibody. Antibodies are major components of the immune system. 

Antibodies are substances made by the body's immune system in response to dangerous and unwanted invaders. Antibodies attach to the foreign substances so the immune system can destroy them.  IgG antibodies are found in all body fluids. They are the smallest but most common antibody (75% to 80%) of all the antibodies in the body.

IgG is found in blood and extracellular fluid allowing it to control infection of the body tissues.  Representing approximately 75% of serum immunoglobulins in humans, IgG is the most abundant antibody isotype found in the circulation. IgG molecules are synthesized and secreted by plasma B cells.

The process by which it disables an invader is called agglutination, which is a form of grouping together of the invaders which immobilises them.  IgG then coats the surface of the resulting immobilised mass with a substance that enables them to be recognised and subsequently ingested by phagocytic immune cells.

IgG antibodies are small, which means that they can not only ‘easily perfuse tissues’, but also pass  through the human placenta, thereby providing protection to the fetus in utero. Along with IgA secreted in the breast milk, residual IgG absorbed through the placenta provides the new born baby with some immunity before its own immune system develops.

There are four IgG subclasses (IgG1, 2, 3, and 4) in humans, named in order of their abundance in serum (IgG1 being the most abundant).  These different subclasses are still being researched and their balancing role is not yet understood.

Clinically, IgG antibody levels are generally considered to be indicative of an individual's immune status to particular pathogens. Thus one way of determining ‘immune status’ in someone is to examine the IgG antibody levels in blood or serum.  For example, a common use of this practice is in tests to measure serologic immunity to measles, mumps, and rubella (MMR), hepatitis B virus, and varicella (chickenpox), among others.