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IgG subclasses: Physical properties, genetics, and biologic functions

Author
Alan P Knutsen, MD
Section Editor
E Richard Stiehm, MD
Deputy Editor
Anna M Feldweg, MD

INTRODUCTION

This topic will review the structure, normal serum concentrations, genetics, and biologic properties of immunoglobulin G (IgG) subclasses. Disorders associated with increased or decreased levels of IgG subclasses are mentioned briefly. IgG subclass deficiency is discussed in detail separately. (See "IgG subclass deficiency".)

PROPERTIES OF IgG SUBCLASSES

IgG is composed of four subclasses: IgG1, IgG2, IgG3, and IgG4 [1-9]. 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 elsewhere. (See "Structure of immunoglobulins" and "Immunoglobulin genetics".)

Structures and physical properties — The four IgG subclasses are present in the serum in monomeric form. The tertiary structures of the IgG subclasses are similar, although they differ in the location and number of interchain-disulfide bonds (figure 1). All four IgG subclasses cross the placenta (table 1) [10-12].

The determination of immunoglobulin class and/or subclass is based upon differences in the heavy chain constant (CH) regions (figure 1). 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) [1].

The Fc region of each IgG subclass has a distinct affinity for phagocyte membrane Fc-gamma-receptors (Fc-gamma-R) (table 1). The binding of IgG molecules to Fc-gamma-Rs is important for initiating phagocytosis. IgG1 and IgG3 bind Fc-gamma-RI, Fc-gamma-RII, and Fc-gamma-RIII, IgG2 binds Fc-gamma-RII, and IgG4 binds Fc-gamma-RI and Fc-gamma-RII [10-13]. (See 'Biologic functions' below.)

            

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Literature review current through: Jul 2017. | This topic last updated: Mar 09, 2017.
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