 In this video, I will describe the role of the primary lymphoid organs, the bone marrow, and the thymus, in lymphocyte development and contrast the functions of the primary lymphoid organs with the functions of secondary lymphoid organs. The major organs of the lymphatic and immune system are called lymphoid organs. The functions of lymphoid organs include housing lymphocytes, regulating their development, and the lymphoid organs clean body fluids. And as they clean the body fluids, lymphocytes within lymphoid organs coordinate the adaptive immunity to defend the body against specific infections. An antigen is any substance capable of binding to the receptors of lymphocytes to provoke an adaptive immune response. You can see here the antibody is a receptor that's found on the surface of a B cell or B lymphocyte when a specific molecular pattern on the surface of a pathogen such as a protein on the surface of a bacterial cell Binds to the antigen binding site of the B cell receptor. This activates the B cell and the B cell can stimulate the adaptive immunity to help defend the body against that specific bacterium, that specific infection. Here we can see an example of a B lymphocyte or B cell becoming activated when the antigen on the surface of a bacterial cell binds to the B cell receptor or antibody. Then this B cell becomes activated to start secreting antibodies that are the pentameric IgM antibodies that will then help to defend against bacteria as the antibody concentration of the blood rises. These antibodies will bind to the bacteria and prevent that bacteria from spreading, will help to neutralize the bacteria and coordinate labeling that bacteria for other leukocytes that come and destroy. Now there's another type of lymphocyte, aside from B lymphocytes, there are also T lymphocytes. This illustration shows us T cell dependent activation of humoral immunity, so this is the activation of a B lymphocyte by a helper T cell. So a T lymphocyte also has a receptor called a T cell receptor which, similar to an antibody, the T cell receptor can bind to an antigen and become activated. And when this T helper cell is becoming activated by the antigen, it's releasing a chemical message called cytokines that will stimulate the B lymphocyte to help activate the B lymphocyte that can then start to produce antibodies. The activated form of the B lymphocyte that secretes the antibodies into the blood are the plasma cells, and so plasma cells are the effector cells in the humoral immunity that secrete antibodies into the blood, and those antibodies can then help to defend against that specific infection. Primary lymphoid organs are the bone marrow and thymus. These organs are the location where lymphocytes mature and become immunocompetent, enabling them to defend the body against infections. An immunocompetent lymphocyte is mature and capable of recognizing an antigen while all lymphocytes are produced from the hematopoietic stem cells in red bone marrow. B lymphocytes remain in red bone marrow to mature, becoming immunocompetent and capable of responding to a specific infection to coordinate the adaptive immunity. Immature T lymphocytes leave the red bone marrow and migrate to the thymus where they complete maturation to become immunocompetent, and so the T cells become immunocompetent in the thymus and thymus begins with T, whereas the B cells become immunocompetent in the bone marrow and bone marrow begins with B. After becoming immunocompetent, mature lymphocytes migrate from the primary lymphoid organs to the secondary lymphoid organs, which are the lymph nodes, the spleen, and the mucosa-associated lymphoid tissues, which include the tonsils as well as numerous patches of lymphoid tissue that are distributed throughout mucous membranes in the body through the digestive tract. Lymph nodes function to filter lymph before it returns to the blood, so lymph forms from interstitial fluid, and interstitial fluid is able to drain into lymphatic capillaries and lymphatic capillaries, then drain into larger lymphatic vessels. These lymphatic vessels will be filtered through lymph nodes before eventually draining into lymphatic ducts that empty into the veins. The lymph nodes contain naïve lymphocytes, that is lymphocytes that are immunocompetent but have not yet become activated by an antigen, and these lymphocytes can then encounter antigen from a pathogen as the lymph is being filtered through a lymph node, and if that antigen can bind to the receptor for that lymphocyte, the lymphocyte will become activated, stimulating the adaptive immunity. These spleen filters and monitors the blood for infection, and so lymphocytes that are in the spleen can help identify infection that's in the blood, and then there are also lymphocytes in the mucosa-associated lymphoid tissues that are able to monitor the mucus and detect any infection in that mucus help to coordinate an adaptive immune response to defend against that infection.