Immunoglobulin G

Immunoglobulin G is primarily involved in the secondary or recall immune response. IgG antibodies are composed of two light chains and two heavy chains. The ability of IgG to diffuse into body tissue facilitates the combination and efficient elimination of antigens. This molecule is further divided into four different subclasses, IgG1, IgG2, IgG3, and IgG4, based on structural differences in the g-heavy chain.

These subclasses also have functional variations in terms of complement fixation, alternative complement pathway activation, macrophage adherence, and ease of placental transfer. All of the subclasses are transferred across the placenta and all of the subclasses partake in antibody-dependent cellular cytotoxicity. IgG1 and IgG3 are the subclasses with the most potent complement fixation abilities. IgG2 also fixes complement, but to a lesser degree, and IgG4 does not activate the classical complement pathway. IgG1 is the most abundant of the subclasses, is the primary subclass responsible for immunity to tetanus/diphtheria, and is also very important in the primary immune response against viral respiratory agents. IgG2 is of primary importance in the body's ability to respond to polysaccharide antigens, such as pneumococcal and meningococcal bacteria. IgG3, along with IgG1, is involved in the primary immune response against viral respiratory agents; IgG3 is the IgG subclass that fixes complement most efficiently. In addition, IgG3 appears to be the primary subclass involved in the antibody response against Moraxella catarrhalis. There is much controversy over the clinical significance of IgG subclass deficiency, but some studies suggest that patients with subclass deficiency are prone to develop sinus and respiratory tract infections. Furthermore, it appears that IgG2 and IgG3 are decreased in 15% to 20% of IgA-deficient individuals. IgG3 is the most commonly deficient subclass in adults, whereas IgG2 deficiency is the most commonly deficient subclass in children.