What are Lymph Nodes?

Lymph nodes are complex, bean shaped and encapsulated organs. The connective tissue capsule encloses the reticular net work packed with lymphocytes, macrophages and dendritic cells.

They are clustered at the junctions of the lymphatic vessels and act as filters of lymph. If pathogens enter the body via respiratory or gastrointestinal tracts they must pass through the regional lymph nodes.

During their passage through the lymph node, pathogens might be trapped by macrophages for phogocytization or initiate immunological response in T and B cells. The size of lymph nodes in humans varies from 1 to 25 mm.

The indentation of the lymph node from which the blood vessels enter and leave the node is known as “hilus”. Several afferent lymphatic ducts carry lymph into the organ, but a single lymph duct with filtered lymph leaves the node from hilus region.

The internal environment of lymph node shows three histological different regions – cortex, para cortex and medulla. Like thymus, at certain regions the outer collagenous layer of capsule penetrates deep into the cortex and paracortex regions ensuing incomplete division of the organ.

The connective tissue partitions are referred as “trabeculae”. The cortex region of lymph node accommodates B cells, macrophages and follicular dendritic cells arranged as primary follicles. Antigenic challenge leads to enlargement of primary follicles into secondary follicles with a germinal center.

Development of two germinal centres indicates intense activation and differentiation of B cells followed by antigenic challenge. Secondary follicles with germinal centres appear more frequently in secondary immune response than in the primary response.

Paracortex next to the cortex region is richly populated by T lymphocytes and inter digitating dendritic cells. The inter digitating cells with high levels of MHC class II molecules (Refer MHC Complex in chapter 11) process the antigens that enter the lymph node and present them to the T cells for further reactions. B cells confine to the edges of the paracortex, and both activated T and B cells confer proliferating foci.

The foci reach maximum size within 3 to 4 days of antigen challenge and some of the B cells differentiate into plasma cells and start producing antibodies. (Refer B cell activation and differentiation in chapter 5). The antibodies bind to the antigens resulting in the formation of antigen antibody complexes.

Follicular dendritic cells trap these antigen antibody complexes and retain for long periods to enhance B cells activation. The dendritic cells with antigen antibody complexes release growth factors that play an important role in B cell activation.

The medulla region of the lymph nodes shows medullary cords formed by connective tissue fibers and sinuses in between them. The medullary sinuses contain plasma cells derived from activated B cells. Phagocytic cells stretch out the margins of medullary sinuses.

When lymph enters into the sub capsular area of the lymphnode through various afferent lymphatic vessels, it slowly percolates in to the cortex, paracortex and medulla, exposing the pathogens moving within, to different immune cells present in the lymph node. Interaction of pathogens with immune cells initiates both humoral and cell mediated immune reactions.

The lymph leaving the lymph node through single efferent lymphatic vessel carries not only lymph but also antibodies and activated lymphocytes produced against the foreign agents after antigenic challenge. The antigenic challenge sometimes initiates migration of lymphocytes from different regions of the body into the lymph nodes.

The increase in the number of lymphocytes results in swelling up of the lymph node. Hence swollen lymph nodes generally indicate presence of pathogens and infection in the body.