Figure 7 IgG Structure
VII. STRUCTURE AND SOME PROPERTIES OF IG CLASSES AND SUBCLASSES
A. IgG
1. Structure
The structures of the IgG subclasses are presented in Figure 7. All IgG's are monomers (7S immunoglobulin). The subclasses differ in the number of disulfide bonds and length of the hinge region.
2. Properties
Most versatile immunoglobulin because it is capable of carrying out all of the functions of immunoglobulin molecules.
a) IgG is the major Ig in serum - 75% of serum Ig is IgG
b) IgG is the major Ig in extra vascular spaces
c) Placental transfer - IgG is the only class of Ig that crosses the placenta. Transfer is mediated by receptor on placental cells for the Fc region of IgG. Not all subclasses cross equally; IgG2 does not cross well.
d) Fixes complement - Not all subclasses fix equally well; IgG4 does not fix complement
e) Binding to cells - Macrophages, monocytes, PMN's and some lymphocytes have Fc receptors for the Fc region of IgG. Not all subclasses bind equally well; IgG2 and IgG4 do not bind to Fc receptors. A consequence of binding to the Fc receptors on PMN's, monocytes and macrophages is that the cell can now internalize the antigen better. The antibody has prepared the antigen for eating by the phagocytic cells. The term opsonin is used to describe substances that enhance phagocytosis. IgG is a good opsonin. Binding of IgG to Fc receptors on other types of cells results in the activation of other functions.
Figure 8 Pentameric serum IgM structure 
Figure 9 Cell
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