2.2 Structure of a lymph node
Lymph nodes are encapsulated organs that are strategically placed along the lymphatic network. Here they can trap foreign material (antigens), which are presented to the lymphocytes by antigen-presenting cells, to initiate an immune response.
The lymphocytes are densely packed in the lymph node, and the tissue is organised both to facilitate the interactions needed to generate an immune response against the antigen and to promote rapid division of the responding lymphocytes.
Lymph nodes vary in size (from a few millimetres to 1–2 cm), and are distributed in different areas of the body. They are linked in chains by lymphatic ducts, so that fluid flowing out of one lymph node via the efferent lymphatic vessel becomes the inflow to the next in line, via the afferent lymphatics.
The fluid in question is called lymph, and is derived from the tissue that carries cells and foreign material to the lymph nodes. Eventually, lymph returns to the bloodstream via one of the body’s two major lymphatic ducts.
Secondary lymphoid organs can be thought of as guard posts that are strategically placed to intercept any infectious agent that enters an area of the body. So, for example, the lymph nodes in the axilla of the arm (the armpit) will intercept infections which enter that part of the body.
Lymphocytes located in the local lymph nodes are responsible for the initial recognition of the infection and the development of the immune response. Once the immune response has developed, the cells will migrate out from the lymph node to the blood and, ultimately, cells will move to the site of infection to combat the pathogen there.
Lymph nodes have a well defined structure with different sub-regions. Different types of leukocyte are localised within the regions so that they can interact with each other appropriately to initiate and develop the immune response. Antigens and cells enter the node through afferent lymphatics, and cells and fluid leave through the efferent lymphatic. Cells (lymphocytes) can also enter the node from the blood by migrating across the specialised high endothelial venules. Within the node, cells distribute themselves to distinct zones. B cells proliferate and develop within the follicles of the cortex, while T cells are primarily located in the paracortex. The capsule, medullary cords and hilus are fixed structural elements of the tissue.
Open thein a new browser window or tab.
Look at Slide 12 in the ‘Week 2’ category within the virtual microscope. This slide is a section from a lymph node that you learned about above.
Now spend a few moments exploring Slide 12, and try to identify some of the structures described. (Don’t forget to use the slide’s legend to help guide your navigation of the sample.)