IgG4

IgG4 is a subclass of the immunoglobulin G (IgG), which is the most common type of antibody found in the circulation of healthy individuals. IgG4 is unique among the IgG subclasses due to its ability to undergo Fab arm exchange and its inability to form large immune complexes.

Structure and Function

IgG4 is a Y-shaped molecule, consisting of two heavy chains and two light chains. The heavy chains are composed of a variable region, which is responsible for antigen recognition, and a constant region, which interacts with effector cells and complement proteins. The light chains also have a variable region and a constant region.

The function of IgG4 is not fully understood, but it is thought to play a role in immune regulation. Unlike other IgG subclasses, IgG4 does not activate the complement system, a group of proteins that work together to destroy foreign invaders. Instead, IgG4 is thought to act as a "blocking" antibody, preventing other antibodies from binding to antigens and triggering an immune response.

Clinical Significance

IgG4 is involved in a number of diseases, including IgG4-related disease (IgG4-RD), a systemic condition characterized by tumefactive lesions and often elevated serum IgG4 concentrations. IgG4-RD can affect any organ system, but commonly involves the pancreas, biliary tract, salivary glands, and lymph nodes.

In addition, IgG4 is implicated in some allergic reactions. In these cases, IgG4 acts as a blocking antibody, preventing IgE from binding to allergens and triggering an allergic response. This is the basis for allergen immunotherapy, in which repeated exposure to small amounts of an allergen is used to increase IgG4 levels and reduce allergic symptoms.

See Also

• Immunoglobulin G
• IgG4-related disease
• Allergen immunotherapy