Immunoglobulin G is an antibody created by the immune system to help fight off infection and disease. This antibody is released by B cells through the immunological synapse in order to destroy viruses, bacteria, or other foreign bodies. The most plentiful of all antibodies, immunoglobulin G is found in all body fluids. Occasionally, it may attack harmless molecules, which sets off an allergic reaction or an autoimmune disorder.
Approximately 75% of immunoglobulins in a normal person's immune system are molecules of immunoglobulin G. Though they are abundant, these antibodies are generally not effective until after the immune system has determined what to use to destroy a certain type of antigen. When the body first encounters an antigen, an immune cell known as a B cell attaches to it and releases antibodies onto its surface. While immunoglobulin G is the most common antibody, it needs to be adapted to destroy each particular type of antigen. Once the body learns how to fight a certain type of antigen, it creates many copies of effective antibodies that then work to eliminate the foreign bodies.
Each molecule of immunoglobulin G is made up of four chains of peptides — two heavy chains and two light chains. These chains are linked together in the middle by strong chemical bonds in a location called the hinge. The hinge is set up so that the four chains branch out in three different directions. The ends of the molecule that are distant from the hinge attach to antigens. Differences in the amino acid chains at the ends allow immunoglobulin G to attack and destroy different types of foreign bodies.
There are a number of types of immunoglobulin G, and levels of each of these types of antibody differ from adult to adult. The types are numbered one through four in order of most to least abundant. The biggest differences between these subclasses are in the type of hinge that the molecule has.
Unlike other antibodies, immunoglobulin G is able to cross the placenta. This makes it instrumental in keeping an unborn baby safe from infection. A fetus acquires this antibody from its mother both through the placenta and through the breast milk, giving a newborn some limited immunity before its own immune system is able to function. A child is able to use its mother's immunoglobulin G until it is about 6 months old, when it is able to create its own.