binding sites. The number of antigen-binding sites is called the antibody’s val-
ence. There are at least two antigen-binding sites on each antigen where a human
antibody can bind. This is referred to as a bivalent antibodybecause the anti-
body’s valence value is two.
The structure of a bivalent antibody is called a monomerand consists of four
protein chains that are named after their relative molecular weights. These are
two light (L) chains and two heavy (H) chains.
These protein chains are joined together to form a Y-shaped molecule that is
flexible enough to form a T-shaped molecule. There are two regions of the pro-
tein chain. These are the variable (V) region and the constant (C) region.
The variable regionis located at the ends of the arms of the Y. These are the
sites where the antibody binds with an antigen. The variable region is a three-
dimensional structure of amino acid sequences whose structure reflects the epi-
topes of the antigen.
The constant regionis the stem of the Yand is called the Fc region. The Fc
region binds adjacent complementary antibodies if both antigen-binding sites
are attached to an antigen; otherwise the adjacent complementary antibodies are
free to attach and react with antigen.
There are five types of constant regions each associated with the five classes
of immunoglobulin.
IgG
IgG immunoglobulinneutralizes bacterial toxins and attacks circulating bacteria
and viruses by enhancing the effectiveness of phyagocytic cells. Nearly 80 per-
cent of the antibodies in serum are IgG. IgG can cross blood vessel walls and the
placenta and can enter tissue fluids.
IgM
IgM immunoglobulinis the first antibody to respond to an antigen or initial infec-
tion and makes up about 10 percent of antibodies in serum. IgM is relatively large
in size and has a pentamer structure of five monomers bonded together by a join-
ing (J) chain. This chain is a polypeptide. Because of its size, IgM remains in
blood vessels and not in tissue fluids. IgM responds to the ABO blood group anti-
gens and enhances the effectiveness of phyagocytic cells. When initial exposure
to an antigen occurs, IgM antibodies are the first to appear.
CHAPTER 14 Immunity^215