Defense Mechanisms of the Body
657
move into lymphatic capillaries and along the lymphatic vessels
to lymph nodes. They may then leave the lymphatic vessels to
take up residence in the node.
This recirculation is going on all the time, not just dur-
ing an infection, although the migration of lymphocytes into
an infl amed area is greatly increased by the chemotaxis pro-
cess (Table 18–3). Lymphocyte traffi cking greatly increases
the likelihood that any given lymphocyte will encounter the
antigen it is specifi cally programmed to recognize. (In con-
trast to the lymphocytes, polymorphonuclear granulocytes
and monocytes do not recirculate; once they leave the blood-
stream to enter a tissue they remain there or die.)
Lymphocyte Origins
The multiple populations and subpopulations of lymphocytes
are summarized in Table 18–1.
B lymphocytes,
or simply
B
cells,
mature in the bone marrow and then are carried by the
blood to the secondary lymphoid organs (
Figure 18–8
). This
process of maturation and migration continues throughout a
person’s life. All generations of lymphocytes that subsequently
Pluripotent stem cell
Myeloid
stem cell
Lymphoid
stem cell
(partial differentiation)
(maturation)
Immature
T cell
Mature
B cell
Bone marrow
(maturation)
Mature
helper
T cell
Thymus
Mature
cytotoxic
T cell
Activation
by antigen
Activation
by antigen
Activation
by antigen
Plasma cells
Antibodies
Secondary lymphoid organs
Figure 18–8
Derivation of B cells and T cells. NK cells are not
shown because their transformations, if any, after
leaving the bone marrow are still not clear.
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