 In this video, I will compare and contrast the origins, maturation process, and general functions of B and T lymphocytes. The adaptive immunity involves lymphocytes that have receptors on their surface, binding to antigens that are molecular patterns associated with specific pathogens. And so the recognition of an antigen by a lymphocyte receptor enables the adaptive immunity to form a specific defense against a particular pathogen and form an immunological memory in order to create a strong defense against that specific pathogen. T lymphocytes are essential for the cell-mediated branch of the adaptive immunity. The receptors on T lymphocytes are known as T cell receptors. An antigen-presenting cell will display antigen associated with a pathogen on its surface, then a T lymphocytes receptor will bind to the antigen that's displayed by an antigen-presenting cell. An antigen-presenting cell could be a cell that engulfed a pathogen by phagocytosis, broke down that pathogen, and then displayed the resulting fragments on the surface as antigens to be displayed for activating the T lymphocytes. Or an individual cell of our body that becomes infected with an intracellular pathogen, like a virus, or it could be a bacteria that grows inside of the cells of our body. But when an infected cell has a pathogen inside, an intracellular pathogen, that infected cell can display antigen from the pathogen on its surface as a way of signaling to T lymphocytes that the cell is infected. So the cell-mediated branch of the adaptive immunity involves effector T cells called cytotoxic T cells. When a T lymphocyte is activated by antigen binding to its T cell receptor, that T lymphocyte will divide to make lots more T lymphocytes that all have the same shape T cell receptor that can be activated by the same antigen associated with a specific pathogen. Some of the T lymphocytes that are produced will be T memory cells providing immunological memory to that specific infection, and some of the T lymphocytes that are produced will be effector T cells. So the effector T cells are cytotoxic T cells that can directly stimulate apoptosis of an infected cell. So if one of our cells becomes infected with a virus, that cell could display antigens from that virus on the surface to activate T lymphocytes and the activated cytotoxic T cells are the effector cells that will release cytotoxic granules containing granzymes and perforants that will then stimulate apoptosis of the infected cell. So this cell mediated immunity will stimulate apoptosis of infected cell in an efficient way to destroy the infected cell without releasing the pathogens that are inside so that any viruses inside of this cell will not be able to infect neighboring cells. B lymphocytes are responsible for the antibody mediated branch of adaptive immunity, which is also known as the humoral immunity. So the term humoral or antibody mediated refers to the fact that the effector B cells known as plasma cells or plasma sites secrete antibodies into body fluids. So the humors are the fluids of the body, the blood, the lymph, and the mucus. And the plasma sites secrete antibodies, and these antibodies can then circulate in the blood or lymph or be secreted onto a mucus membrane in the mucus, where the antibodies can then bind to antigens on the surface of a pathogen in order to help direct the immune defenses to protect against that specific pathogen. So while the effector cells of the antibody mediated immunity are a type of B lymphocyte, we will see that some T lymphocytes are involved in the antibody mediated immunity. These are known as T helper cells that can help stimulate the activation of B lymphocytes. So a B cell or B lymphocyte is activated when an antigen binds to the B cell receptor, which is an antibody on the surface of the plasma membrane of the B cell. It will require a repetitive antigen to bind multiple B cell receptors to have T cell independent activation of B lymphocytes. But if we have this type of situation, as is shown in the illustration, where a pathogenic bacterial cell has a repetitive antigen on its surface, the B lymphocyte will bind with its B cell receptors to the antigens and become activated. And then that B cell will divide to produce more B lymphocytes that all have the same shaped antibodies. Some of these B cells will be memory B cells to provide an immunological memory to that specific pathogen. And some of the B cells produced are the effector B cells, the plasma cells that secrete antibodies into the blood, lymph and mucus in order to help provide the antibody mediated defense of the adaptive immunity. And we'll see that antibodies could bind to a bacterial cell to neutralize the bacteria, could bind to a virus that's extracellular in order to neutralize that virus, or antibodies can also tag a pathogen in order to direct the process of phagocytosis. Antibodies can label a pathogen in order to direct the phagocytic leukocytes to come in and gulf and break down that pathogen. Antibodies can also direct other defense proteins such as the complement system to help defend against specific infections. All lymphocytes are produced in the mechanism of hematopoiesis within red bone marrow. While natural killer cells are a type of lymphocyte important for the innate immunity, the T lymphocytes and B lymphocytes are important for the adaptive immunity, while T lymphocytes leave the red bone marrow as immature T lymphocytes and migrate to the thymus to complete positive and negative selection to produce the immunocompetent T lymphocytes. B lymphocytes will complete maturation within red bone marrow. So an immature B lymphocyte first goes through a process of positive selection where lymphocytes with functional receptors are stimulated to continue their maturation and apoptosis removes lymphocytes with receptors that are not functional. Following this positive selection is a process of negative selection that stimulates apoptosis in lymphocytes that have receptors capable of binding to self antigens. Self antigens are molecular patterns associated with the cells of our own body and negative selection eliminates those lymphocytes that would cause an autoimmune disease where the adaptive immune system starts to attack antigens produced by our own body. So here we can see the shape of an antibody and a B lymphocyte or B cell that has a antibody on its surface an IgM or IgD antibody on the surface of the B cell functions as the B cell receptor that can bind to an antigen and this can then stimulate the activation of the B cell to produce more B lymphocytes that all have the same shaped B cell receptors as well as the effector B cells that will secrete antibodies that also have the same variable region so the same shaped antigen binding site while antibodies function as the B cell receptors enabling B lymphocytes to recognize antigens associated with specific pathogens a different protein the T cell receptor is present on the surface of the plasma membrane of T lymphocytes and enables T lymphocytes to become activated by specific antigens while a repetitive antigen can activate B lymphocytes independent from any requirement of T lymphocyte activation most antigens will require helper T cells to complete the activation of the antibody mediated immunity so the T cell dependent activation of antibody mediated immunity will start with the B cell receptor binding to the antigen on the surface of a pathogen like a bacterium then that B cell will engulf the pathogen or bring that pathogen in through phagocytosis break down the pathogen and display on its surface antigens from the pathogen so the B cell will function as an antigen presenting cell presenting the antigen from this pathogen to a helper T cell when the helper T cells activated by the antigen displayed from the B cell the helper T cell will secrete cytokines that complete the activation of the B cell so this juxtocrine signaling mechanism will enable the completion of B cell activation then this activated B cell will divide go through cell proliferation to make many more B lymphocytes that all produce antibodies that can bind to the same antigen some of these will be the long-lived memory B cells and some of these will be the effector plasma cells that secrete antibodies these secreted antibodies then will function for the antibody mediated immunity binding to antigens on the surface of the pathogen and helping to defend against infection so while adaptive immunity is not as quick to respond as the innate immunity after a initial exposure the adaptive immunity will develop an immunological memory enabling a rapid response to a second exposure to the same pathogen so it can take a couple of weeks for a primary immune response to develop to the initial exposure we can see over that time the concentration of antibody gradually rises and then antibody mediated immunity will clear the infection and eventually the concentration of antibody in the blood will decrease after the infection has been cleared from the body but it won't go all the way back down to the very low initial levels it won't go back down to essentially zero instead there will be a constantly elevated level of antibodies that can defend against that specific pathogen and this is what provides immunity to a subsequent exposure so that when you're exposed to the same pathogen a second time the antibodies that are already circulating can help defend against that infection and the memory B cells that also have antibodies capable of binding to antigens on the surface of that pathogen will become activated and those memory B cells will start to divide producing more B lymphocytes including the effector B cells the plasma cells that secrete antibodies to ramp up the immune defense the antibody mediated immunity T cell maturation occurs within the thymus after immature T lymphocytes are produced in red bone marrow they migrate to the thymus where they go through positive and negative selection so an immature T lymphocyte an immature T cell is also known as a thymocyte and the double negative thymocytes are the immature T lymphocytes that migrate from red bone marrow into the thymus within the outer layers of the thymus the cortex of the thymus there are epithelial cells that have MHC proteins on their surface in the process of positive selection the thymocytes that have T cell receptors that can bind to the MHC proteins on thymic epithelial cells will be stimulated to progress further through maturation and in this process they will be stimulated to produce two cell surface proteins known as CD4 and CD8 if a thymocyte T cell receptor cannot bind to the MHC protein on the surface of a thymic epithelial cell that thymocyte will be stimulated to enter apoptotic cell death so that in this mechanism of positive selection only thymocytes with functional T cell receptors capable of binding to MHC proteins will be able to progress through positive selection and move on to the next step of T cell maturation known as negative selection negative selection occurs deeper within the thymus in the medulla of the thymus within the medulla of the thymus dendritic cells display self antigen from the body cells using MHC proteins to display these antigens if the T cell receptor binds to a self antigen displayed by the dendritic cell during negative selection that cell will be stimulated to enter apoptosis and die in this way apoptotic cell death will remove the T lymphocytes that could recognize antigens produced by our own body and so negative selection will help prevent autoimmune diseases and so that only T lymphocytes that cannot be activated by self antigens will continue through the negative selection process completing their maturation to become immunocompetent lymphocytes so at the beginning of negative selection the T lymphocytes express both CD4 and CD8 proteins but a cell signal at the end of negative selection will activate a T lymphocyte to either become a CD4 positive CD8 negative mature T helper cell a cell that has a CD4 protein but does not have a CD8 protein so the CD4 protein will work with the T cell receptor on a T helper cell and this will recognize the MHC2 protein in contrast the other thymocytes the other T lymphocytes that mature will have the CD8 T cell receptor complex but not the CD4 protein so these are CD4 negative CD8 positive thymocytes that mature becoming cytotoxic T cells that have the CD8 T cell receptor complex which can be activated by the MHC1 protein the mature lymphocytes will then migrate out of the thymus and into the secondary lymphoid organs like the lymph nodes, the spleen, and the mucosa associated lymphoid tissues antigen presentation is important for the mechanism of T cell maturation as well as T cell activation all the cells in our body can function as antigen presenting cells when they become infected with intracellular pathogens cells will use an MHC1 protein to display antigens from that pathogen on the surface leading to activation of cytotoxic T cells that have the CD8 T cell receptor complex the cytotoxic T cell will become activated and then release cytotoxic granules activating the mechanism of apoptotic cell death in the infected cell in contrast there are professional antigen presenting cells which are phagocytic cells so the major phagocytic leukocytes like macrophages, monocytes, neutrophils, as well as dendritic cells as is shown in this illustration and also the B lymphocytes all can function as professional antigen presenting cells so a professional antigen presenting cell will engulf a pathogen by phagocytosis then the lysosome containing digestive enzymes will merge with the the vacuole produced in phagocytosis and will break down the pathogen and fragments from that pathogen will then be packaged into MHC2 proteins and displayed on the surface of the cell the MHC2 protein displayed by a professional antigen presenting cell will then activate a T helper cell which is a T cell that has a CD4 T cell receptor complex so here we can see the mechanisms of T cell activation on top is the mechanism of T cell activation by a professional antigen presenting cell that is a phagocytic cell that engulfs a pathogen and then displays antigen from that pathogen on its surface using an MHC2 protein a T helper cell that has a CD4 positive T cell receptor complex can bind to the antigen displayed by the MHC2 protein on the surface of a professional antigen presenting cell and this leads to activation of the helper T cell and an activated helper T cell then releases cytokines that will stimulate activation of other lymphocytes in the adaptive immunity on the bottom we see activation of a cytotoxic T cell by an infected cell so for example a cell of our body infected by a virus or some other intracellular pathogen can lead to the production of MHC1 proteins with antigen from that pathogen so the infected cell is functioning as an antigen presenting cell displaying antigen from its infection using an MHC1 protein and this MHC1 antigen complex will activate the CD8 T cell receptor complex on the surface of a cytotoxic T cell leading to activation of that cytotoxic T cell, stimulating it to release cytotoxic granules that contain perforins and granzymes leading to apoptotic cell death