 This video will cover the following objective from the lymphatic and immune system. List the major functions of the lymphatic and immune system. Describe how lymph is formed and transported through lymphatic vessels, lymph nodes, and lymphatic ducts. Compare and contrast the origins, maturation process, and general function of B and T lymphocytes. The major functions of the lymphatic system include maintaining the blood volume by transporting liquid that leaks out of blood capillaries in the process of filtration. That liquid that leaks out becomes interstitial fluid and then interstitial fluid is transported by the lymphatic system back to the blood. Interstitial fluid, as it accumulates, will drain into lymphatic capillaries that have endothelial flaps that function as one-way valves in order to prevent backward flow of the lymph to keep the lymph moving into larger and larger lymphatic vessels, eventually into the lymphatic ducts that will drain lymph into the blood at the subclavian veins. The organs of the lymphatic and immune system include the lymphatic vessels and lymph nodes that filter lymph as it's flowing through. There are lymph nodes clustered on the lymphatic vessels that contain lots of leukocytes, especially lymphocytes, that are involved in the filtering of the lymph to filter out any debris or any pathogens and to recognize any infection, any pathogens and mount an immune response to help protect against infection. The leukocytes defend against infection. They can migrate out of the blood and are attracted by chemical signals that are produced at the site of an infection. In the process of emigration, leukocytes migrate through the endothelium of the capillary and migrate into a tissue to help defend against infection. The leukocytes can release cytotoxic granules that kill the invading pathogenic cells. Another function that leukocytes could use to defend against infection is phagocytosis. Phagocytosis, a leukocyte engulfs a pathogen and then that pathogen will be broken down inside as the pathogen is brought in. It forms a vacuole and the vacuole will merge with a lysosome containing digestive enzymes that will kill the pathogen and break down the pathogen. The defense function of the immune system includes both an innate and an adaptive immunity. An innate immunity refers to defenses that you are born with including the surface defenses such as your skin, hair and mucus that help to create a physical barrier to prevent infection. It also includes internal defenses such as the leukocytes, the mass cells and basophils release inflammatory signals like histamine that will help to increase blood flow to the tissue and increase the permeability of capillaries and attract other leukocytes to the scene of an infection. Natural killer cells are a special type of lymphocyte that's part of the innate immunity that can release cytotoxic granules that will kill cells that have become infected with a virus. So the natural killer cells will kill our own cells when our cells become infected with a virus. Then the complement system refers to defense proteins that can punch holes in the cell walls of bacteria to kill those cells. Then the phagocytic cells, the monocytes, neutrophils and macrophages. Macrophages are a leukocyte that forms from a monocyte so it's basically a type of monocyte after the monocyte exits the blood, travels into its tissue and differentiates into a macrophage. What monocytes, neutrophils and macrophages are all phagocytic cells that can engulf pathogens and then degrade those pathogens. And so while these are all part of the innate immunity that you're born with these defenses, the adaptive immunity recognizes specific pathogens and forms a memory to those specific infections. The lymphocytes are the primary cells of the adaptive immunity and there are two major branches of the adaptive immunity. The T lymphocytes are responsible for the cell mediated branch of the adaptive immunity whereas the B lymphocytes are responsible for the antibody mediated branch of the adaptive immunity. Both T cells and B cells have receptors that bind to antigens. So an antigen is a specific molecular pattern and that molecular pattern is on the surface of a pathogen like a bacterial cell or on the surface of a virus is how the adaptive immunity will recognize that specific pathogen. And so the body forms an enormous variety of different T lymphocytes and B lymphocytes with distinct shapes of receptors that can bind an enormous variety of antigens. And when our body encounters an antigen from an infection, we'll make more B lymphocytes and T lymphocytes that can defend against that specific pathogen by binding to the antigens that are on the surface of that pathogen. The cell mediated immunity will involve cytotoxic T cells as the effector cells that will release cytotoxic granules to kill our own cells if our own cells become infected with intracellular pathogens such as a virus. The cytotoxic T cells will recognize antigens from the virus and then become activated and release cytotoxic granules that kill our own viral infected cells. And so the cytotoxic T cell functions similar to the natural killer cell. They're both lymphocytes, but the natural killer cell is part of the innate immunity and does not recognize specific viruses, whereas the cytotoxic T cells will defend against specific viruses by recognizing the antigens from that virus. B lymphocytes produce the antibodies as part of antibody mediated immunity or the humoral immunity, and the antibodies are secreted by a type of B lymphocyte known as a plasma cell or a plasma site. These antibodies then travel in the blood and lymph and can bind to antigens on the surface of pathogens. Then the antibody functions to help defend against that infection by neutralizing the pathogen and sensitizing the other defense mechanisms that can come and destroy that pathogen. So the antibodies will effectively target the pathogen to stimulate other leukocytes to come and destroy that pathogen. Another function of the lymphatic system is to transport lipid soluble nutrients like fat and cholesterol and the fat soluble vitamins, vitamin A, vitamin D, vitamin E, and vitamin K from the small intestine where these nutrients are absorbed. When lipid soluble nutrients are absorbed in the small intestine, they're absorbed as large droplets, lipid droplets that are coated with proteins, which are known as lipoprotein particles, the specific lipoprotein particles that are produced in the small intestine during lipid nutrient absorption are called chylomicrons. These chylomicron lipoprotein particles are too large to enter into the blood capillaries in the small intestine, and so they'll enter into lacteals, which are larger vessels. They're lymphatic vessels. Lacteals are essentially a specialized lymphatic capillary that is responsible for absorbing the chylomicron lipoprotein particles, enabling the absorption of dietary lipids like fats, cholesterol, and fat soluble vitamins. Lymph flows from lymph capillaries to larger lymph vessels, then it flows through lymph nodes along the lymph vessels where it is filtered, and then lymph flows from larger lymph vessels into lymphatic ducts, which are the largest lymphatic vessels. The largest lymphatic vessels called lymphatic ducts are the right lymphatic duct and the thoracic duct. The right lymphatic duct drains lymph coming from the upper right half of the body superior to the diaphragm, whereas the thoracic duct drains lymph from the majority of the body, the entire region shown in yellow on the illustration, which is the left half of the body, as well as the right half of the body inferior to the diaphragm. Lymph from this entire region then flows through the thoracic duct into the left subclavian vein to return the fluid to the blood, whereas the right lymphatic duct drains into the right subclavian vein. Lymphocytes are the major cells of the adaptive immunity, although there is one type of lymphocyte that's not involved in the adaptive immunity, and instead is part of the innate immunity called the natural killer cell. The T lymphocytes and B lymphocytes are the two major cell types that are important for the adaptive immunity. While T lymphocytes are involved in the cell mediated immunity, the B lymphocytes are involved in antibody mediated immunity. However, there is some crosstalk so that T lymphocytes can stimulate the B lymphocytes to promote the antibody mediated immunity. There are a type of T lymphocyte known as a helper T cell that will stimulate the B lymphocytes. The T lymphocytes have the effector cells known as cytotoxic T cells that carry out the cell mediated immunity to protect against viral infections and other intracellular pathogens. The T lymphocytes mature in the thymus, which is a lymphoid organ located in the superior mediastinum just anterior to the base of the heart. While all of the lymphocytes originally are formed in the process of hematopoiesis in red bone marrow, immature T lymphocytes migrate from the red bone marrow to the thymus where they become fully matured to become what we call immunocompetent. An immunocompetent lymphocyte is a lymphocyte that is prepared to bind antigens and become activated to help defend against infections. The B lymphocytes, they finish becoming immunocompetent, they fully mature within the red bone marrow before they leave the red bone marrow. The T lymphocytes become immunocompetent in the thymus and B lymphocytes become immunocompetent in the bone marrow. So here we see the two major branches of the adaptive immunity. The cell mediated immunity involves the T lymphocytes. When the receptors of T lymphocytes bind an antigen, the T lymphocyte becomes activated and will divide making more T lymphocytes that produce the same receptor and can recognize the same antigen. Some of these cells will be long-lived memory T cells whereas other cells will be the effector T cells, the cytotoxic T cells that respond to release cytotoxic granules destroying our own cells that have become infected with a pathogen. In the antibody mediated immunity, the B lymphocytes are the major lymphocyte that's activated. The receptors of B lymphocytes are called antibodies, so there's an antibody in the plasma membrane on the surface of the B lymphocyte. When that receptor binds an antigen, the B lymphocyte becomes activated and divides to produce both some long-lasting memory B cells as well as the effector B cells. The effector B cells are called plasma cells or plasma sites. They secrete antibody into the lymph and blood plasma and the antibodies then can travel in the fluids of the body and bind to pathogens, neutralizing those pathogens and marking the pathogens to sensitize other defense mechanisms that will come and destroy the pathogen. Clonal selection refers to the proliferation of lymphocytes to produce more daughter cells that all have the exact same receptor that can be activated by the same antigen. Clonal selection of T cells occurs when a naive T cell that has not encountered an antigen yet first encounters an antigen. The antigen binds to the T cell receptor and the T cell receptor then stimulates activation of the T cell. So this naive T cell has become activated and then it will divide to make more T cells and all of the cells that are produced when this cell divides will have the same T cell receptor. And some of these daughter cells will immediately function as the effector cytotoxic T cells that can destroy infected cells in the body. And some of the T lymphocytes that are produced from the division of this activated T cell will be the memory T cells that are long lived and can help to defend against the same infection if it's encountered again in the future. The B cell receptors are antibodies. The antibodies are embedded in the plasma membrane of B cells to function as the B cell receptor. However, antibodies are also secreted by the effector B lymphocytes, the plasma cells. Clonal selection of B cells occurs when antigen binds to the receptor of the B lymphocyte. If a naive B lymphocyte has not been activated yet, once the antigen binds to its receptor, the B lymphocyte becomes activated and is no longer a naive B lymphocyte. An activated B lymphocyte then will divide. It will proliferate to make many more B lymphocytes that all can produce the same shaped antibody to recognize the same antigen. Some of these cells that are produced from division of the activated B cell are the plasma cells, the effector cells that secrete antibodies. And some of them are memory B cells that are long lived in order to help defend against exposure to the same pathogen in the future. The presence of memory B cells will enable a secondary exposure to the pathogen to stimulate a much faster and larger production of antibodies as there are many memory B cells that are prepared and ready to be activated if the antigen is encountered. That antigen will then stimulate the memory B cells to divide and produce plasma cells, producing a strong response to that secondary exposure. Here we see the basic time course of the adaptive immunity with a primary immune response following the initial exposure to a pathogen. The first time the immune system has encountered that antigen, the concentration of antibody in the blood rises slowly from a very low level over time. It will take a couple weeks for the antibody level to rise to a maximum and then gradually fall after the infection is cleared from the body. However, the memory cells will be produced and have a long lasting effect if the same antigen is encountered again on a secondary exposure. The secondary immune response will be much more rapid and will have a much stronger effect leading to a much higher concentration of antibody in the blood to produce a strong protective adaptive immunity. Ideally, the adaptive immunity is so effective to a secondary exposure that you don't develop any symptoms because you're immune to that infection. This is how vaccinations work. In a vaccination, you're given an initial exposure to the antigen using an inactivated form of a virus, usually. It could be an inactivated form of an antigen from a bacterial cell as well, but mostly for viruses we use vaccination. This will stimulate the primary immune response and then if you are exposed to that virus in the future, the secondary immune response should be able to prevent you from developing any symptoms of infection. Thank you for watching.