There are four immunoglobulin G subclasses, each numbered for their relative abundance in the body. These molecules are an important part of the immune system. All immunoglobulin G molecules are made up of four chains of amino acids, connected at a central hinge in such a way that three ends extend from the center. The subclasses differ in only about 5% of their amino acids, and it is the differences in the hinge connecting the four chains that are responsible for the structural and functional differences between them.
Of the four immunoglobulin G subclasses, immunoglobulin G1 is by far the most abundant. This molecule makes up more than half of the total immunoglobulin G in the human body and more than a quarter of the amount of immunoglobulin of any class. It is able to quickly and easily bind to foreign proteins, making it an effective antibody. Immunoglobulin G1 is also easily able to cross the placental barrier, which makes it instrumental in the creation of a temporary immune system in a newborn.
Immunoglobulin G2 is the second most abundant of the immunoglobulin G subclasses. It is the smallest of these types of antibodies and also the most inflexible because there are more connections within the molecule's hinge. Though significantly more abundant than the G3 or G4 subclasses, this antibody does the least amount of damage to antigens. Though it is not as effective as other antibodies in its class, the short length of the hinge makes it less susceptible to damage from molecules that could harm it.
The hinge within the immunoglobulin G3 antibody is by far the longest in all the immunoglobulin G subclasses. This region of the molecule contains 62 amino acids, compared with 15 in the next longest hinge, which is found in immunoglobulin G1. The length of the hinge allows immunoglobulin G3 to quickly and effectively bind to and eliminate antigens. On the other hand, the length of the hinge also makes this subclass of immunoglobulin G the most susceptible to damage from other molecules.
The least abundant of the immunoglobulin G subclasses is immunoglobulin G4. This antibody has the same sized hinge as immunoglobulin G2 but lacks the extra chemical bonds that make immunoglobulin G2 particularly rigid. It is the most common antibody in this class to respond to allergens, though it can bind to viruses and bacteria, as well.