Page 851 - Fundamentals of anatomy physiology

838 Unit 4 Fluids and Transport
Figure 22–23 Antibody Structure and Function.

Antigen binding Heavy chain Antigen binding
site site

Light Antigenic
chain determinant

sites

Disulfide
bond

Variable
segment

Complement Antigen Antibodies
binding site
Constant Site of binding
segments to macrophages

of light
and heavy

chains

a A diagrammatic view of the structure of an antibody. c Antibodies bind to portions of an antigen called
antigenic determinant sites, or epitopes.

Light chain

Antigen
binding site

22 Complete
antigen

Heavy chain +
b A computer-generated image of a typical antibody.
Hapten Carrier
molecule

d Antibody molecules can bind a hapten (partial

antigen) once it has become a complete antigen

by combining with a carrier molecule.

Antibodies do not bind to the entire antigen. Instead, they become attached to carrier molecules, forming combinations
bind to specific portions of its exposed surface—regions called that can function as complete antigens (Figure 22–23d). In
antigenic determinant sites, or epitopes (Figure 22–23c). some cases, the carrier contributes an antigenic determinant
The specificity of the binding depends initially on the three- site. Antibodies will then attack both the hapten and the car-
dimensional “fit” between the variable segments of the anti- rier molecule. If the carrier molecule is normally present in
body molecule and the corresponding antigenic determinant the tissues, the antibodies may begin attacking and destroying
sites. A complete antigen has at least two antigenic deter- normal cells. This process is the basis for several drug reactions,
minant sites, one for each of the antigen binding sites on an including allergies to penicillin.
antibody molecule. Exposure to a complete antigen can lead to
B cell sensitization and a subsequent immune response. Most Classes and Actions of Antibodies
environmental antigens have multiple antigenic determinant
sites, and entire microorganisms may have thousands. Body fluids may contain five classes of antibodies, or immu-
noglobulins (Igs): IgG, IgE, IgD, IgM, and IgA (Table 22–1).
Haptens, or partial antigens, do not ordinarily cause B cell The classes are determined by differences in the structure of the
activation and antibody production. Haptens include short heavy-chain constant segments. For this reason, the classes have
peptide chains, steroids and other lipids, and several drugs, no effect on the antibody’s specificity, which is determined by
including antibiotics such as penicillin. However, haptens may the antigen binding sites.