The lymphatic vessels drain interstitial fluid or lymph from peripheral tissues back into the blood.
Lymphoid tissue and organs contain a lot of lymphocytes and other white blood cells.
The primary lymphoid organs include the thymus and bone marrow.
And the secondary lymphoid organs include the tonsils, lymph nodes, spleen and mucosa-associated lymphoid tissue or MALT for short.
Lymph nodes are small secondary lymphoid organs that are found along lymphatic vessels throughout the body.
They’re encapsulated, bean-shaped structures that usually have a diameter of about 1 cm along the short axis and 2.5 cm along the long axis.
And they support the immune system by filtering the lymph, in order to identify and fight infections.
If we zoom closer, we can more easily identify the outer capsule of connective tissue, as well as the three functional regions of the lymph node.
Just beneath the capsule is the outer cortex, which contains spherical nodules or follicles of B cells, each with a germinal center, similar to the follicles of the spleen.
The next region is the inner cortex or paracortex, which doesn’t have any nodules.
And finally, the innermost region of the lymph node is the medulla.
The distinction between the inner cortex and medulla is hard to see at this magnification, but the medulla will have cords of lymphoid tissue, as well as passageways for lymph called medullary sinuses.
These sinuses eventually join one another and drain into the efferent lymphatic vessels.