Defense Mechanisms of the Body
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.)
The multiple populations and subpopulations of lymphocytes
are summarized in Table 18–1.
mature in the bone marrow and then are carried by the
blood to the secondary lymphoid organs (
process of maturation and migration continues throughout a
person’s life. All generations of lymphocytes that subsequently
Pluripotent stem cell
Secondary lymphoid organs
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.