Chapter 13
Innate and Adaptive Immunity
301
foreign material from lymph before it enters the bloodstream
and serving as centers for proliferation and response of
immune cells.
Lymph nodes are bean-shaped, encapsulated tissues,
approximately 0.5 to 1 cm in diameter. Lymph enters the node
through afferent lymph channels and leaves through the effer-
ent lymph vessels located in the deep indentation of the hilus.
Lymphocytes and macrophages move slowly through the
lymph nodes so that they have adequate time to engulf micro-
organisms and interact with circulating antigen. The lymphatic
system provides a large surface upon which macrophages and
DCs can more easily present antigens to T lymphocytes.
Lymph nodes are divided into three distinct and spe-
cialized areas—an outer cortex, a paracortex, and an inner
medulla (Fig. 13.14). The T lymphocytes predominate in the
paracortex and the B lymphocytes predominate in the follicles
and germinal centers of the outer cortex. The T lymphocytes
proliferate when antigens enter the paracortex of the lymph
node. They then migrate to the outer cortex so that they can
interact with B lymphocytes that are stored there. Within the
follicles the lymphocytes continue to mature, replicate, and
interact with the PACs present in the nodes (macrophages and
follicular DCs). Activated B cells then migrate to the medulla
of the lymph node, where they complete their maturation into
plasma cells. Large quantities of antibodies are then released
into the systemic circulation.
in the cortex under the influence of thymic hormones and
cytokines. As the T cells mature, they develop the TCRs that
differentiate them from other types of T cells. The majority
of the thymocytes die in the cortex during the process of gene
rearrangement and maturation because they fail to develop
the appropriate receptor types on their cell membranes. Only
those T cells capable of recognizing foreign antigen displayed
by self-MHC are allowed to mature. This process is called
thymic selection
. Mature, immunocompetent T-helper and
T-cytotoxic cells leave the thymus in 2 to 3 days and enter the
peripheral lymphoid tissues through the bloodstream.
Lymph Nodes
Lymph nodes are small aggregates of lymphoid tissue located
along lymphatic vessels throughout the body. The lymphatic
vessels carry lymph, which is a clear sometimes yellow-
tinged fluid that contains a variety of white blood cells (pre-
dominantly lymphocytes) and transports cellular debris and
organisms to the lymph modes to be removed from the body.
Each lymph node processes lymph from a discrete, adjacent
anatomic site. Lymph nodes are congregated in the axillae
and groin and along the great vessels of the neck, thorax,
and abdomen. The lymph nodes receive lymph from the col-
lecting ducts, which ultimately drain into the thoracic duct
located in the left side of the chest at the level of the sub-
clavian vein. Lymph nodes have two functions: removal of
FIGURE 13.14
•
Structural features of a lymph
node. Bacteria that gain entry to the body are filtered
out of the lymph as it flows through the node.
Afferent
vessel
Medulla
Paracortex
Follicles
Cortex
Reticular
fibers
Lymphoid
sinuses
Nodule
Efferent vessel
Medullary cord
Capsule
Germinal
center
Trabecula
Hilus
