IgG subclasses: Physical properties, genetics, and biologic functions
- Alan P Knutsen, MD
Alan P Knutsen, MD
- Director, Pediatric Allergy and Immunology
- Director, Jeffery Modell Center for Primary Immunodeficiencies
- Professor of Pediatrics
- St. Louis University Health Sciences Center
IgG can be subdivided into four subclasses: IgG1, IgG2, IgG3, and IgG4 [1-5]. This topic will review the structure, normal serum concentrations, genetics, and biologic properties of IgG subclasses. Disorders associated with increased or decreased levels of IgG subclasses are mentioned briefly and discussed in detail separately. (See "IgG subclass deficiency".)
PROPERTIES OF IgG SUBCLASSES
The structure, genetics, and function of the IgG subclasses are reviewed in this section. Detailed discussions of these topics as they apply to immunoglobulins in general are presented separately. (See "Structure of immunoglobulins" and "Immunoglobulin genetics".)
Structures and physical properties — Each of the four IgG subclasses is present in the serum in monomeric form. The tertiary structure of the molecules is similar, although they differ in the location and number of interchain-disulfide bonds (figure 1). All four IgG subclasses cross the placenta.
The determination of immunoglobulin class and/or subclass is based upon differences in the heavy chain constant (CH) regions (figure 2). Functional differences among the subclasses arise from structural variation in both the Fc regions and in the hinge regions (the area where the variable or Fab regions are joined to the Fc region) .
The Fc region of each IgG subclass has a distinct affinity for phagocyte membrane Fc receptors. The binding of IgG molecules to these receptors is important for initiating phagocytosis. IgG1 and IgG3 bind Fc receptors more effectively than IgG2 and IgG4 . (See 'Biological functions' below.)
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- PROPERTIES OF IgG SUBCLASSES
- Structures and physical properties
- Normal serum concentrations
- BIOLOGICAL FUNCTIONS
- Complement activation
- Role in specific types of infections
- DISORDERS ASSOCIATED WITH ELEVATED SUBCLASS LEVELS
- DISORDERS ASSOCIATED WITH REDUCED SUBCLASS LEVELS