Immunoglobulin G (IgG)

Immunoglobulin G is the prototype of antibody
molecules produced by derivatives of B lymphocytes,
the plasma cells. The molecule has
two H chains and two L chains, held together by
disulfide bonds. Each H chain has three constant
regions (CH1, CH2, and CH3) and one variable
region (VH). Each H chain has a total of 446
amino acids, of which the first 109 belong to the
variable region at the N-terminal end. Each L
chain has one variable (VL) and one constant
(CL) domain and consists of 214 amino acids. In
the L chains also, the first 109 amino acids form
the variable region. The variable domains of the
H chains and the L chains form the antigenbinding
sites. The three hypervariable regions
within the V region of each chain are also called
complementarity determining regions (CDR)
because the actual physical contact of
molecules based on their complementary
structure occurs in these regions. Each domain
consists of about 110 amino acid residues. A
jointlike area (hinge) between constant region 1
(CH1) and constant region 2 (CH2) of the heavy
chain allows some flexibility of the molecule.
The H chains are bound to each other and the H
to the L chains by disulfide bridges (—S—S—).
Furthermore, there are disulfide bridges within
the constant and variable domains. The L chains
are of one of two types, ! or !. In addition to immunoglobulin
G, there are other types of immunoglobulins,
which differ from each other in
the constant part of the H chain: IgA (C"), IgD
(C#), and IgE (C$). A very large immunoglobulin,
IgM, is made up of five IgG subunits. The different
types of H chains are referred to as isotypes.