In this video we will discuss on lymphoid
organs. But before proceeding to this video, please watch the videos on Lymphatic vessels and
formation of lymph, for better understanding. The links to videos are given in the description. Let’s move on to the lymphoid organs.
These organs provide immunity for the body It is divided into primary lymphoid
organs and secondary lymphoid organs. Primary lymphoid organs are bone marrow and
thymus. These are organs where immune cells, namely T lymphocytes, and B lymphocytes
are produced. B lymphocyte mature within the bone marrow. Whereas the immature T
lymphocytes migrate to thymus for maturation. Secondary lymphoid organs are, spleen, tonsil,
lymph nodes, and Mucosa associated lymphoid tissue, called MALT, appendix, and peyer’s
patch of small intestine. After the maturation of lymphocytes in the primary lymphoid organ,
lymphocytes relocate to these secondary lymphoid organs, where they are activated by antigens, and
fight against the germs and foreign substances. Let’s now concentrate on anatomy of lymph nodes.
Lymph nodes are kidney shaped organ, measuring 1 to 2 cm in size depending on the location it
is present in the body. It has concave hilum, and a convex surface. The convex surface,
receives multiple afferent lymphatic vessels. After the filtration of lymph, in the lymph node,
the lymph exits the lymph node via the one or two efferent lymphatic vessels, at the concave
hilum. Lymph node has an arterial supply, which enters via the hilum. Within the lymph
nodes, the artery divides into capillaries, then the capillaries drain into
venules, then venules drain into a vein. There is a specialized swelling of the
post capillary venules in the lymph node, these swellings are called high endothelial
venule. High endothelial venules, are the sites of trans endothelial migration of
circulating lymphocytes into the lymph nodes. High endothelial venules are present in all
secondary lymphoid organs, except for the spleen. Let’s discuss the lymph node structure.
It consists of capsule, subcapsular sinus, outer cortex, and inner medulla. We will
discuss one by one. Capsule of lymph node is dense connective tissue stroma and collagenous
fiber. The capsule extends into the centre of the lymph node in the form of trabeculae.
Beneath the capsule is the subcapsular sinus. Subcapsular sinus is the space between the capsule
and the cortex. Afferent lymphatic vessels drain into the subcapsular sinus. Lymph that received in
the subcapsular sinus, flows along the trabecular sinuses and finally drain into medullary
sinuses, in the medulla of the lymph node. From the medullary sinuses, the lymph exits the
lymph node via the efferent lymphatic vessels. Beneath the subcapsular sinus, is the cortex. Cortex again divided into outer cortex and inner
cortex. Inner cortex is also called paracortex. Outer cortex consists of B lymphocytes,
these are arranged in follicles. So, outer cortex is also called B cell layer.
The paracortex consists mainly of T lymphocytes. So, inner cortex or the paracortex is also
called T cell layer. When the B cells of the outer cortex are exposed to antigen, they
are activated, and form germinal center. Following this, resting B-cell and dendritic
cells surround the germinal center to form a mantle zone. Medulla is the innermost layer of the
lymph node. It contains, blood vessels, medullary sinuses, and medullary cords. The medullary cords
contain antibody secreting plasma cells, B-cells and macrophages. Medullary sinuses separate the
medullary cords. Medullary sinuses are spaces within the medulla, and it receives lymph from
the trabecular sinuses and subcapsular sinuses. The medullary sinus drains the lymph
into the efferent lymphatic vessels. The main function of lymph node is
filtering the lymph collected from tissues, and returning it to the circulatory system.
During this filtering action of lymph node, the B cells and T cells are exposed to a variety of
antigens. This plays a role in adaptive immunity. This completes the structure
and function of the lymph node. Hope the video was useful. See
you soon in the next video.