Lymph nodes are small, bean-shaped organs that possess afferent and efferent lymphatic vessels, allowing them to function as in-line filters for lymph that drain from tissues and organs throughout the body. Lymph nodes filter lymph and provide a site for presentation of antigens that are carried in the lymph to B and T cells, promoting their activation and mounting an immune response to foreign substances, cells, and microorganisms. (Click here for a larger image of the lymph node.)

Examine sections of lymph nodes (sample 1, sample 2) and images below. First, note the overall organization of the lymph nodes (cortex containing lymphatic nodules, paracortex, and medulla).

At higher magnification, examine these structures: subcapsular sinus, lymphoid follicles (also called lymphatic nodules), high endothelial venules, and medullary cords and sinuses. As you find these structures, consider the path of lymph flow. Lymph enters the lymph node through afferent lymphatic vessels that pass through the capsule, it then flows into the subcapsular sinus, along the cortical or trabecular sinuses, into the medullary sinuses, and out through the efferent lymphatic vessels. As lymph flows through the node it brings antigens in close contact with B and T cells and antigen-presenting cells in the lymphatic nodules and medullary cords. (Click here for a larger image of lymph node detail to the right. Click here for printable version of image below.)

Finally, take a look at this silver stained slide of lymph node to see the reticular connective tissue that supports the lymphocytes and other immune cells within the lymph nodes.

Clinical note: Lymphomas involve neoplastic growth of lymphocytes, commonly within lymph nodes. There are several types, depending on the type of lymphocyte and the stage of differentiation involved. In the early stage, lymphomas commonly involve cell accumulation caused by their failure to undergo apoptosis. Pictured here is a child with Burkitt's lymphoma.

Now for specifics on the tonsils.