Lymph
Nodes
Lymph
nodes are distributed throughout the body along the course of the lymphatic
vessels. The nodes are found in the axilla and the groin, along the great
vessels of the neck, and in large numbers in the thorax and abdomen, especially
in mesenteries. Lymph nodes constitute a series of in-line filters that are
important in the body's defense against microorganisms and the spread of tumor cells.
All this lymph, derived from tissue fluid, is filtered by at least one node
before returning to the circulation.
Gross
Features of Lymph Nodes
Lymph nodes are elongated or kidney-shaped organs that
have a convex surface that is the entrance site of lymphatic vessels and a
concave depression, the hilum, through which arteries and nerves enter
and veins and lymphatic vessels leave the organ (Figure- 1). A connective
tissue capsule surrounds the lymph node, sending trabeculae into its
interior.
Structure
of Lymph Node
The
most common cells of lymph nodes are lymphocytes, macrophages and other APCs,
plasma cells, and reticular cells; follicular dendritic cells are present
within the lymphoid nodules. The lymph node consists of two regions:
an
outer cortex and
an
inner medulla
The
cortex can further be subdivided into an outer cortex and an inner
cortex or paracortical region.
Cortex
1. A diffuse population of cells
composed mainly of T lymphocytes and reticular cells; macrophages and APCs are
also present in this area.
2. Lymphoid nodules, with or
without germinative centers, formed mainly by B lymphocytes, embedded in the
diffuse population of cortical cells.
3. Areas of loose lymphoid tissue
(whose reticular fibril meshes are wide) situated immediately beneath the
capsule, called the subcapsular sinuses. They are composed of a loose
network of reticular cells and fibers. Lymph, containing antigens, lymphocytes,
and APCs, circulates around the wide spaces of these sinuses after being
delivered into these channels by the afferent lymphatic vessels.
4. Intermediate or radial
sinuses that run between lymphoid nodules. These sinuses arise from and
share the same structure with the subcapsular sinuses. They communicate with
the subcapsular sinuses through spaces similar to those present in the medulla.
The inner cortex
or paracortical region does not have precise boundaries with the outer cortex
and contains few, if any, nodules but many T lymphocytes.
Medulla
The medulla has
two components:
1. Medullary Cords and
2. Medullary sinuses
1. Medullary Cords
The medullary cords are
branched cordlike extensions of dense lymphoid tissue that arise in the inner
cortex.
They contain primarily B
lymphocytes and often plasma cells and macrophages.
2. Medullary Sinuses
The
medullary cords are separated by dilated spaces, frequently bridged by
reticular cells and fibers, called the medullary sinuses.
They
contain lymph, lymphocytes, often many macrophages, and sometimes even
granulocytes if the lymph node is draining an infected region. These sinuses
(which arise from the intermediate sinuses) join at the hilum delivering the
lymph to the efferent lymph vessel of the lymph node.
Lymph
Circulation through Lymph Node
Afferent
lymphatic vessels (Fig-1)
cross the capsule and pour lymph into the subcapsular sinus. From there, lymph
passes through the intermediate sinuses and, finally, into the medullary
sinuses. During this passage, the lymph infiltrates the cortex and the medullary
cords. The lymph is finally collected by efferent lymphatic vessels (Fig-1) at
the hilum.
Valves in both
the afferent and efferent vessels aid the unidirectional flow of lymph.
Role of Lymph
Nodes in the Immune Response
Because lymph
nodes are distributed throughout the body, lymph formed in tissues must cross
at least one node before entering the bloodstream. The lymph that arrives at a
lymph node may contain antigens, either soluble molecules, portions of
semidestroyed microorganisms, or antigens already internalized and being
transported by macrophages and other APCs. It may also contain cytokines and
other cells (such as neutrophils and eosinophils), particularly if it is coming
from a region undergoing inflammation. The antigens that had not been phagocytosed
before may be internalized by APCs of the lymph nodes. All antigens have the
opportunity to be presented to B lymphocytes and to T helper and T cytotoxic
lymphocytes, to initiate an immune response.
The lymph node is
an important site of lymphocyte proliferation (for instance, of B cells in the
germinative centers) as well as of transformation of B lymphocytes into plasma
cells. Because of this, the lymph that leaves a lymph node may be enriched in
antibodies. As the lymph is transported to veins, these antibodies will
ultimately be delivered to the entire body by the blood circulation.
Recirculation
of Lymphocytes
Because all lymph
formed in the body drains back into the blood, lymphocytes that leave the lymph
nodes by efferent lymphatic vessels eventually reach the bloodstream. They may
then leave the blood vessels by entering the tissues and return to another
lymph node by a lymph vessel. They may also return to a lymph node (a process
called homing) by crossing the walls of specific blood vessels, the high
endothelial venules (HEVs), present in lymph nodes.
The continuous
recirculation of lymphocytes enables most parts of the body to be constantly
monitored, increasing the opportunity for lymphocytes to encounter APCs and
antigens that have migrated to lymph nodes.
High
endothelial venules (HEVs)
These venules have
an unusual endothelial lining of tall cuboidal cells. L-selectin present on the
lymphocyte surface recognizes sugar-rich ligands of the endothelial cell
surface, and as a consequence, the lymphocyte stops in the internal wall of the
vein. Integrins are probably important for the adhesion of the lymphocytes to
the endothelial cells and the lymphocytes eventually cross the vessel wall into
the lymph node parenchyma. High endothelial venules are also present in other
lymphoid organs, such as the appendix, tonsils, and Peyer's patches, but not in
the spleen.
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