What Are IgG Subclasses?

Human immunoglobulins are specific protein molecules that help the body fight off an infection, usually a recurring exposure to a specific foreign body, or antigen. Each immunoglobulin (or Ig) has a different protein structure that allows it to respond to a specific antigen. There are five classes of immunoglobulins: IgG, IgA, IgD, IgE and IgM. Of these five classes, IgG makes up approximately 70 to 75 percent of the total antibodies in humans. IgG has four different subclasses, identified as IgG1, IgG2, IgG3 and IgG4.

IgG Subclass Distribution

• The relative serum blood concentrations of the four IgG subclasses differ between each variety. IgG1 predominates, followed by IgG2. The relative serum concentrations of IgG3 and IgG4 are about equal, but are found in very small quantities.

IgG Subclass Function

• The different subclasses of IgG provide flexibility for the individual to respond to a foreign organism, such as a virus or bacteria. All four IgG subclasses have small, discrete differences in their protein structure that allow them to respond to a presented foreign antigen. Once these antibodies are activated, they help the body destroy antigens by marking them with a specific protein coat. The body then engulfs them with a cell-eating macrophage and they are destroyed.

IgG1 and IgG2 Subclass Deficiency

• Several different illnesses are thought to be caused by a deficiency of either IgG or IgG subclasses. Since IgG1 is the most abundant IgG subclass, an insufficient amount of this antibody type may lead to a general reduction in immune response, making the individual susceptible to recurrent infections. A deficiency of IgG2 may lead to a lowered response to encapsulated (protein covered) antigens, lowering the immune response to these organisms.

IgG3 and IgG4 Subclass Deficiency

• Although found in minute quantities, IgG3 is very important to the overall immune response. Therefore, reduced production of IgG3 can also leave an individual susceptible to recurring infections. The IgG4 subclass deficiency is harder to quantify. All IgG subclasses are able to cross the placental barrier; this allows a mother to confer immunity to a newborn. However, IgG4 may not reach maturation levels until the child is two or three years old. A change in maturation level of the individual may change the levels of IgG4 production.