Maintenance of the Body
maturation process in the thymus. B cells become immuno-
competent and self-tolerant in the bone marrow. Te lymphoid
organs where the lymphocytes become immunocompetent—
thymus and bone marrow—are called
primary lymphoid
. All other lymphoid organs are referred to as
lymphoid organs
Te selection process (education) that lymphocytes undergo
is best understood in ± cells, so we will explore it first. ± cell edu-
cation consists of positive and negative selection
(Figure 21.9)
both of which occur in the thymus.
Positive selection
is the first of two tests a developing ±
lymphocyte must pass. It ensures that
± cells that are
able to recognize self-MHC proteins survive. Remember
that ± cells cannot bind antigens unless the antigens are
presented on self-MHC proteins. ± cells that are unable to
recognize self-MHC are eliminated by apoptosis.
Negative selection
, the second test, ensures that ± cells do
not recognize self-antigens displayed on self-MHC. If they
do, they are eliminated by apoptosis. Negative selection
is the basis for immunologic self-tolerance, making sure
that ± cells don’t attack the body’s own cells, which would
cause autoimmune disorders.
Tis education of ± cells is expensive indeed—only about 2% of
± cells survive it and continue to become successful immuno-
competent, self-tolerant ± cells.
Less is known about the factors that control B cell matura-
tion, but a similar selection process is thought to occur in the
bone marrow. B cells that successfully make antigen receptors
are positively selected and those whose antigen receptors are
self-reactive are eliminated (
clonal deletion
). However, selection
is not perfect for either B or ± cells—some self-reactive lym-
phocytes survive.
Seeding Secondary Lymphoid Organs and Circulation (Fig-
ure 21.8
Immunocompetent B and ± cells that have not
yet been exposed to antigen are called
. Naive B cells and
± cells are exported from the primary lymphoid organs to seed
(colonize) the secondary lymphoid organs—lymph nodes,
spleen, and so on—where they are likely to encounter antigens.
Lymphocytes, especially the ± cells (which account for 65–85%
of bloodborne lymphocytes), circulate continuously throughout
the body. Circulating greatly increases a lymphocyte’s chance of
coming into contact with antigens located in different parts of the
body, as well as with huge numbers of macrophages and other
lymphocytes. For example, a ± cell circulates through lymph and
blood and back to a lymph node about once a day. Although lym-
phocyte circulation appears to be random, lymphocyte movement
into the tissues is highly specific, regulated by homing signals
(CAMs) displayed on vascular endothelial cells.
Antigen Encounter and Activation (Figure 21.8
Te first
encounter between an immunocompetent but naive lymphocyte
and an invading antigen usually takes place in a lymph node or
the spleen, but it may happen in any secondary lymphoid organ.
Immune cells in lymph nodes are in a strategic position to en-
counter a large variety of antigens because lymphatic capillaries
pick up proteins and pathogens from nearly all body tissues.
of receptor on their surface. Tese receptors (some 10
cell) enable the lymphocyte to recognize and bind a specific
antigen. Once these receptors appear, the lymphocyte is com-
mitted to react to one (and only one) distinct antigenic deter-
minant because
of its antigen receptors are the same. Te
receptors on B cells are in fact membrane-bound antibodies.
Te receptors on ± cells are not antibodies but are products of
the same gene superfamily and have similar functions.
Each lymphocyte must be relatively unre-
sponsive to self-antigens so that it does not attack the body’s
own cells. Tis is called
B and ± cells acquire immunocompetence and self-tolerance in
different parts of the body. ± cells undergo this two- to three-day
Adaptive defenses
T cells
recognize self major histocompatibility proteins (self-MHC)
thymic cell
Failure to recognize self-
MHC results in
(death by cell suicide).
Recognizing self-MHC
results in survival.
Survivors proceed
to negative selection.
Recognizing self-antigen
results in
. This
eliminates self-reactive
T cells that could cause
autoimmune diseases.
Failure to recognize (bind
tightly to) self-antigen
results in survival and
continued maturation.
T cell receptor
T cell
Cellular immunity
T cells
must not
recognize self-antigens
1. Positive Selection
2. Negative Selection
Figure 21.9
T cell education in the thymus.
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