Diversity of Antibodies

It is estimated that there are about 100 million trillion (1020) antibody molecules in each individual, representing a few million different specificities for different antigens. Considering that antibodies that bind to particular antigens can cross-react with closely related antigens to some extent, this tremendous antibody diversity usually ensures that there will be some antibodies that can combine with almost any antigen a person might encounter. These observations evoke a question that has long fascinated scientists: How can a few million different antibodies be

Antigen molecule

ab re9lon

Fc region ab re9lon

Fc region

■ Figure 15.10 The structure of antibodies. Antibodies are composed of four polypeptide chains—two are heavy (H) and two are light (L). (a) A computer-generated model of antibody structure. (b) A simplified diagram showing the constant and variable regions. (The variable regions are abbreviated V, and the constant regions are abbreviated C.) Antigens combine with the variable regions. Each antibody molecule is divided into an Fab (antigen-binding) fragment and an Fc (crystallizable) fragment.

Light chain of antibody

Antigen

Antigen-Antibody Complex

Light chain of antibody

Antigen

(a) Heavy chain of antibody (b)

Antigen-Antibody Complex

(a) Heavy chain of antibody (b)

■ Figure 15.11 The antigen-binding site of an antibody. The structure of the Fab portion of an antibody molecule and the antigen with which it combines as determined by X-ray diffraction. (a) The heavy and light chains of the antibody are shown in blue and yellow, respectively, and the antigen is shown in green. Note the complementary shape at the region where the two join together in (b).

Reprinted with permission from A. G. Amit, "Three Dimensional Structure of an Antigen-Antibody Complex at 2.8 A Resolution" in Science, vol. 233, August 15, 1986. Copyright © 1986 American Association for the Advancement of Science.

produced? A person cannot possibly inherit a correspondingly large number of genes devoted to antibody production.

Two mechanisms have been proposed to explain antibody diversity. First, since different combinations of heavy and light chains can produce different antibody specificities, a person does not have to inherit a million different genes to code for a million different antibodies. If a few hundred genes code for different H chains and a few hundred code for different L chains, different combinations of these polypeptide chains could produce millions of different antibodies. The number of possible combinations is made even greater by the fact that different segments of DNA code for different segments of the heavy and light chains. Three segments in the antigen-combining region of a heavy chain and two in a light chain are coded by different segments of DNA and can be combined in different ways to make an antibody molecule.

Second, the diversity of antibodies could increase during development if, when some lymphocytes divided, the progeny received antibody genes that had been slightly altered by mutations. Such mutations are called somatic mutations because they occur in body cells rather than in sperm cells or ova. Antibody diversity would thus increase with age as the lymphocyte population increased.

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