We have hundreds of lymph nodes in our body. These bean-shaped structures are secondary lymphoid organs. Remember that bone marrow and thymus are the primary lymphoid organs.
A lymph node is a cluster of various kinds of cells. These cells, in turn, perform various functions that are attributed to the lymph node:
- Macrophages that perform non-specific filtration of the lymph. Macrophages are part of the innate arm of the immune system hence their function is non-specific.
- B and T cells. These cells are part of the acquired arm of the immune system.
- Follicular dendritic cells that help refine the B cell’s function.
Lymph nodes are encapsulated. Their outer region is called cortex. Cortex is covered by a fibrous capsule that sends extensions inwards. These extensions are called trabeculae. Region internal to the cortex is called medulla.
AFFERENT AND EFFERENT CHANNELS
There are many afferent channels bringing lymph from tissues to a lymph node, and efferent channel that brings the lymph back to the heart via the thoracic duct.
The outer region is called superficial cortex. It contains B cell follicles.
The inner region of the cortex comes in contact with medulla and is called paracortex. Paracortex contains T cells.
The superficial cortex contains B cell clusters. These clusters are called follicles.
Clusters of densely packed inactive B cells appear dark under a microscope. These clusters are called primary follicles.
During the infections that activate B cells, the active B cells in the center of the follicles appear relatively light. Follicles with pale germinal centers are called secondary follicle.
This is the outer part of a secondary follicle. It contains small lymphocytes wrapping around an active germinal center. This is the area of lymphoma in mantle cell lymphoma.
In the follicles, special dendritic cells are also present. These follicular dendritic cells present antigens to B cells. This activity allows B cells to refine their antigen binding sites. This process of immune response maturity is called affinity maturation.
This region is sandwiched between the superficial cortex and medulla. It contains T cells. This region is unique that it has high endothelial venules. These venules allow T and B cells from the blood to enter the lymph node tissue.
The paracortical region is less developed in DiGeorge Syndrome.
During the active infections (usually viral) the paracortical region swells due to extreme T cell proliferation.
Consists of cords that contain T cells and activated B cells (plasma cells). There are spaces between the cords called medullary sinuses. These sinuses are lined with reticular cells and macrophages. Lymph from these sinuses drains into the efferent lymph channels to be sent back to the cardiovascular system.