Introduction 
THYMUS 
Precursors 
Hemopoietic 
Stem Cell 
CD3/TCR/S 
o 
/ 
Epithelial 
Stromal CD3/TCRa/3 
Cells 
CDS/TCR/S 
^(~\- ySTCell 
— ^rnn + 
Cytotoxic T Cell 
\ CD3/TCRa/3 X^-^CD8- 
(J) (^^^^^f^-=^ ^ CD3/TCR 0/3 
CD4,8+ Helper T Cell 
CD4' 
^~.CD8± 
IgM + lgD igM 
a— o^(s>^ 
Natural Killer Cell 
Antibodies 
Stromal Cell 
sIgM IgM+lgD+IgG, 2 30,4 '9Gi,2.3or4 
cllis^ ^ \\lgm+lgD+lgA,<„2 IgA, or 
Fetal Liver 
Bone Marrow 
igM 
igG, 
2.3 or 4 
IgM+lgD+lgE 
IgA, or 2 
Mature B Cells 
Plasma Cells 
Figure 17. Contemporary model of immune system development in mammals. T and B cells, which form the major 
recognition and effector limbs of the immune system, are derived from multipotent stem cells that also give rise to the 
other types of blood cells. Definition of the T cell receptors and accessory cell surface molecules has allowed the 
identification of two major sublineages of the thymus-derived T cells, each of which expresses a different type of T cell 
receptor and exhibits specialized function. The B lineage cells, generated in hemopoietic tissues, initially express one 
class of antibody receptors called immunoglobulin M ( IgM ), but may then switch to the expression of other antibody 
classes, each of which has special biological advantages. Note the third lineage of lymphocytes, the natural killer 
cells. They are the most recently recognized members of the lymphocyte family, but may well prove to be the most 
phylogenetically ancient elements in this constellation of cells involved in host defense. 
Courtesy of Max D. Cooper. 
