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.

The outer cortex, situated under the capsule, consists of the following components:
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.

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.


Junqueira's Basic Histology: Text and Atlas, 13th Edition

Wheater's Functional Histology: A Text and Color Atlas, 5th Edition