Antibody Classes

Immunoglobulins (Ig) in the five classes and subclasses can be expressed either as secreted immunoglobulins (sIg) or as membrane bound immunoglobulins (mIg). sIg has a hydrophillic amino acid sequence of various lengths at the C-terminus whereas the mIg carboxyl terminal domain contains three regions:

1. an extracellular hydrophillic "spacer" sequence composed of 26 amino acid residues
2. a hydrophobic transmembrane sequence
3. a short cytoplasmic tail

The immature pre B-cells express only mIgM on their surfaces. mIgD appears during maturation and is co-expressed with IgM on the surface of the mature B-cell. Even when different classes of immunoglobulins are expressed sequentially on a single cell, the antigenic specificity of all the membrane antibody molecules are identical so that each antibody molecule binds to the same epitope.

Immunoglobulin G (IgG)

IgG (Kuby et al.)

All IgGs are monomers (7S immunoglobulin). Their subclasses differ in the number of disulfide bonds and the length of the hinge region.

Properties

• IgG is the most versatile immunoglobulin because it is capable of carrying out all of the functions of immunoglobulin molecules
• It is the major Ig in serum (75% of serum Ig is IgG) and in extra vascular spaces
• It is the only class of Ig that crosses the placenta. The transfer is mediated by receptors on placental cells for the Fc region of IgG. IgG1, IgG3 and IgG4 readily cross the placenta and protect the developing fetus.
• Complement activation - IgG3 is the most effective followed by IgG1. IgG2 is less efficient while IgG4 does not fix complement at all.
• Binding to cells - Macrophages, monocytes and some lymphocytes have Fc receptors (FcR) for the Fc region of IgG. IgG4 has intermediate affinity and IgG2 has very low affinity. A consequence of binding to the Fc receptors on monocytes and macrophages is that the cells can internalize the antigens better. 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).