Introduction 
Vh segments (hundreds) 
D segments ( 20) 
Jh segments (4) 
Germ-line 
organization 
Vh 
First rearrangement 
JhI 
y N 
Second rearrangement 
— » 
Vh| 
Vh|d| Jh 
|jh 
N N 
Figure 15. The specificity of antibodies and the receptors on T cells is brought about by a series of complex 
rearrangements of various gene segments like those shown in this figure. 
From Molecular Cell Biology, 2nd edition. James Darnell, Harvey Lodish, and David Baltimore, p. 1027. 
Copyright ® 1990 by Scientific American Books, Inc. Reprinted with permission by W.H. Freeman and 
Company. 
Light chains 
Antigen-binding 
site 
Antigen-binding 
site 
Variable regions 
Antigen-binding 
site 
Light chain 
Hypervoriable 
regions 
Figure 16. A: A molecular model of an antibody mol- 
ecule. Each amino acid is represented by a small 
sphere. The heavy chains are colored blue, the light 
chains red. The four chains wind about one another 
to form a Y shape, with two identical antigen-bind- 
ing sites at the arms of the Y and a tail region that 
serves to direct the antibody to a particular portion 
of the immune response. 
B: A schematic drawing of an antibody molecule. 
Each molecule is composed of two identical light (L ) 
chains and two identical heavy (H) chains. Carbo- 
hydrate is sometimes associated with the H chain. 
While the antigen-binding sites are formed by a com- 
plex of both H and L chains, the tail region is formed 
by H chains alone. 
From Raven, P.H., and Johnson, G.B. 1988. Under- 
standing Biology. St. Louis, MO: Times Mirror/Mosby 
College Publishing, p. 692. 
xliv 
