Introduction 
Figure 18. A schematic representation of 
how the thymus gland selects T cells that are 
able to recognize foreign but not self 
proteins. 
From Molecular Cell Biology, 2nd edition. 
James Darnell, Harvey Lodish, and David 
Baltimore, p. 1037. Copyright ® 1990 by 
Scientific American Books, Inc. Reprinted 
with permission by W.H. Freeman and 
Company. 
Bone-marrow cells 
Rearrange gene segments to make 
a unique T-cell receptor (TCR) 
for the cell surface 
TCR 
Thymus 
gland 
Into the 
selection 
system 
Rejected: TCR binds to 
self-MHC plus self-peptides 
(negative selection) 
Rejected: TCR does not bind 
tightly enough to self-MHC 
(positive selection) 
Accepted (-5% of cells): TCR 
appropriate to recognize foreign 
peptides bound to self-MHC 
j I To peripheral lymphoid 
y J system 
