 This video will cover the following objective from the lymphatic and immune system. Describe the structure, location, and function of lymphatic organs, lymph nodes, spleen, tonsils, pyres, patches, thymus, and bone marrow. This illustration shows us the major lymphoid organs, the bone marrow, and the thymus are known as the primary lymphoid organs, whereas the rest of the lymphoid organs, like the spleen and the lymph nodes, are secondary lymphoid organs. Any lymphoid organ or lymphoid tissue is a cluster of leukocytes that's contained within a connective tissue framework. Typically, there's a loose reticular connective tissue framework where the leukocytes are found and this is surrounded by a capsule of fibrous, dense connective tissue. The primary lymphoid organs are the location where lymphocytes mature to become immunocompetent, that is, where they become capable of becoming activated by an antigen. The red bone marrow is the primary lymphoid organ that's found in the trabecular cavities of spongy bone, and the thymus is the other primary lymphoid organ. The thymus is located in the superior mediastinum anterior to the base of the heart, while the red bone marrow is the location where B lymphocytes become immunocompetent. The thymus is the location where T lymphocytes become immunocompetent. This picture shows us the red bone marrow within trabecular cavities of spongy bone. This is from the proximal head, the proximal epiphysis of the femur, the head of the femur. There is spongy bone filling the head of the femur, containing red bone marrow, and within red bone marrow, there are the hemocytoblasts or hematopoietic stem cells that divide and differentiate to produce all of the formed elements of blood, but this is also the location where B lymphocytes become immunocompetent. This illustration is showing us the process of hematopoiesis, where the hemocytoblast divides to produce more multipotent stem cells that are capable of differentiating into any of the formed elements, and then the hemocytoblasts can differentiate to form the myeloid stem cell or the lymphoid stem cell. The myeloid stem cell forms the majority of the formed elements, all formed elements that are not lymphocytes or natural killer cells, but the the lymphoid stem cell can differentiate to form the lymphocytes, including the T lymphocytes that are responsible for the cell-mediated immunity, producing the effector cytotoxic T cells, and the B lymphocytes that are responsible for the antibody-mediated adaptive immunity, where the effector B lymphocytes are plasma cells that secrete antibodies. The thymus is located within the mediastinum, just posterior to the sternum, anterior to the base of the heart. The thymus is the location where T lymphocytes mature to become immunocompetent, and the majority of this function is occurring during early life, so the thymus is larger in newborns and children and becomes smaller as we get older. The T lymphocytes are first formed in the red bone marrow, and then they migrate into the thymus to become the thymocytes. These immature thymocytes then go through a pop, a process called positive selection, where the thymocytes with functional T cell receptors are stimulated to mature further, and then the T lymphocytes go through a negative selection process, which will stimulate the death of T lymphocytes that have a receptor that could recognize self-antigens, and this negative selection process helps to prevent autoimmune diseases. Lymph nodes are small kidney-shaped organs clustered along lymph vessels that function to filter lymph. Lymph flows in through afferent vessels into the loose, reticular-connective framework or stroma of the lymph node, and the lymph flows through the cortex and then through the medulla and then out through the efferent lymphatic vessels. As lymph flows through the lymph node, the leukocytes found within the stroma can recognize an infection and help to defend against infection. There are numerous lymphocytes found within the germinal centers. These germinal centers form follicles within the cortex of the lymph node, where the lymphocytes will rapidly divide during the process of clonal selection to produce more lymphocytes that can all defend against the same antigen. And then the effector B lymphocytes migrate from the cortex of the lymph node into the medulla in structures known as medullary cords that consist of the reticular-connective tissue, containing leukocytes, especially the plasma cells, the effector cells of the antibody-mediated immunity, will cluster in the medullary cords and secrete antibodies that can flow out through the efferent lymphatic vessels. So here we see a low magnification view of the lymph node with the outer layer is formed out of fibrous connective tissue that is folding inward to form trabeculae that separate the cortex into lymph follicles, round structures that are filled with reticular-connective tissue and leukocytes. And the lymph follicles are the location where activated lymphocytes will rapidly divide. Then deeper in the medulla of the lymph node, we can see the purple staining on the medullary cords. Medullary cords are also consisting of reticular-connective tissue and leukocytes. The medullary cords contain the plasma cells that are secreting antibodies for the antibody-mediated branch of the adaptive immunity. The spleen is a large lymphoid organ located in the left hypochondriac region. The function of the spleen is to filter the blood. Leucocytes within the spleen can remove worn-out erythrocytes in order to degrade worn-out erythrocytes. And the spleen also can filter out pathogens and help to respond to defend against infection. Within the spleen that the structure is an outer layer of fibrous capsule that then surrounds a stroma of reticular-connective tissue and numerous leukocytes are found within that stroma. The white pulp is regions of the stroma that contains a large amount of lymphocytes. And this white pulp is arranged surrounding the central arterioles where blood is delivered into the capillaries, the sinusoidal capillaries of the spleen. And then the red pulp is found surrounding the sinusoids and veins. The red pulp contains macrophages that can engulf worn-out erythrocytes. And so the numerous erythrocytes that are found in red pulp give red pulp a red appearance under the microscope. And so here we see the histology of the spleen with an outer capsule of fibrous connective tissue surrounding an inner stroma of loose reticular connective tissue forming the red pulp where there's numerous sinusoids and veins that will venules that will drain into the veins to carry blood out of the spleen. And in the red pulp there are macrophages that are involved in performing phagocytosis to remove worn-out erythrocytes. And so numerous erythrocytes give a red appearance to the red pulp. And then we can see a darker bluish staining region is the white pulp of the stroma in the spleen. And the white pulp contains numerous lymphocytes that are activated and dividing. So the white pulp is the location where lymphocytes will divide and the blood flows in to the spleen through arteries that branch into central arterioles that are located within the white pulp. So the white pulp is where the blood enters into a segment of the spleen and then it flows through the sinusoids and drains out through veins at the red pulp. There are lymphoid tissues found in mucous membranes known as the mucosa associated lymphoid tissues abbreviated malt. And the tonsils are the largest examples of mucosa associated lymphoid tissues. There is a large tonsil located in the superior region of the throat. The superior region of the throat is known as the nasopharynx. So pharynx is the word for throat and nasopharynx is the part of the throat that connects to the nasal cavity. The pharyngeal tonsil is found in the roof of the nasopharynx where it is helping to filter mucous and filter out any particles and pathogens that are entering the nasal cavity. Then the palatine tonsils are found at the back of the oral cavity at the border of the pharynx between the oral cavity and the oral pharynx. You can see in the picture on the bottom here the palatine tonsils are swollen due to infection. And so the function of the palatine tonsils is to filter the mucous that's entering the pharynx from the oral cavity and it will these will monitor for infection and in response to any infection can become inflamed and enlarged. Sometimes the palatine tonsils are removed in a tonsilectomy procedure in order to help open up the pharynx if the palatine tonsils are becoming so enlarged that it's difficult to breathe. There's also a lingual tonsil in the base of the tongue. And so the lingual tonsil is also functioning to filter the mucous in the oral cavity and monitor for infection. There's also mucosa associated lymphoid tissue found in the intestines known as pyres patches. These are clusters of lymphoid tissue that are found in the mucous membrane known as the mucosa in the inner wall of the intestine. These pyres patches could be found throughout the intestine, but they're most numerous in the most distal region of the small intestine, the part that drains into the large intestine known as the ilium and they're also found at high concentration in the appendix. And so the function of pyres patches is to filter the mucous in the intestines and monitor for infection and help to help to prevent infection and regulate the bacterial environment of the intestines. Here we can see another image of the histology of the intestine showing the pyres patch located in the ilium region of the small intestine. The pyres patches are found in the mucosa and are monitoring for infection in the small intestine, helping to protect against infection that could spread through the small intestine and helping to regulate the bacterial environment of the small intestine.