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Lymph Nodes
Lymph nodes are examples of the secondary, or peripheral, immune organs,
which are the sites where the adaptive immune responses to foreign antigen
are initiated. The human body contains many hundred lymph nodes situated
at various points in the lymphatic system (lymphatics). They are rich in both
lymphocytes and antigen presenting cells (APCs) and so provide an environment
where immune responses to antigen in the lymph may be triggered and develop.
They thus act as filters of the lymph before it returns to the blood, capturing
and responding to foreign antigen that have entered the body via portals of entry
such as the skin [14,13].
Lymph nodes are small bean shaped structures connected to the lymphatics
via a number of afferent lymph vessels through which lymph enters the node,
and a single efferent lymph vessel through which the lymph leaves the node.
Each lymph node is also connected to the circulatory system via a lymphatic
artery and vein. It is through the lymphatic artery that lymphocytes (mainly
naive T and B cells) enter the lymph node. As lymph drains though the node,
any antigen present is captured and processed by APCs for presentation to
lymphocytes, which consequently initiates the chain of events that results in
the adaptive immune response. Antigen may also be transported into the lymph
node by APCs, called dendritic cells, that have captured the antigen close to the
portal of entry and then migrated to the node via the lymphatics.
The lymph node can be functionally separated into three distinct areas each
supporting a different cellular environment: the cortex, the paracortex and the
medulla. The cortex supports supports mainly B cells and various APCs
(macrophages and dendritic cells), the paracortex supports mainly naive T
H
cells and dendritic cells, and the medulla contains mostly lymphocytes including
the antibody producing plasma cells. As lymph drains through the lymph node,
it slowly percolates though each of these three regions. In the paracortex, the
dendritic cells trap and process any foreign antigen and presents it via MHC-
II to the naive T
H
cells resulting in their activation. These T
H
cells then play
their part in activating B cells on the edge of the paracortex leading to B cell
proliferation. This proliferation takes place in the germinal centres of the cortex,
and results in antibody producing plasma cells, some of which migrate to the
medulla. This whole process results in the lymph leaving the lymph node being
enriched with antibodies and lymphocytes [14].
The segmentation of the lymph node into the three different areas is due to the
presence of a particular variety of signalling molecules called chemokines. Both
naive T
H
cells and dendritic cells activated due to exposure to antigen, express
the same cell-surface receptor for a chemokine produced only in the paracortex.
This has the effect of attracting both of these cell types into the same area,
thus enabling their interaction. Likewise, naive B cells are concentrated in the
cortex as they express a receptor for a different chemokine produced only in the
cortex. Once T
H
and B cells have been activated by antigen/APCs, they lose
their chemokine receptors from the cell surface, and therefore migrate towards
each other. Thus the structure of the lymph node keeps each of the T and B
