 In this video, I will describe the events of cell-mediated immunity, describe the role of antigen-presenting cells, helper T cells, and cytotoxic T cells in the cell-mediated response. The helper T cells are T lymphocytes that express the CD4 positive form of the T cell receptor complex and are activated by antigen, displayed in the MHC2 protein by professional antigen-presenting cells such as dendritic cells, macrophages, monocytes, neutrophils, or B lymphocytes that have engulfed a pathogen by phagocytosis, and then broken down that pathogen, the fragments from the pathogen are displayed on the surface of the professional antigen-presenting cell using an MHC2 protein. The T cell receptor CD4 complex will bind to the antigen MHC2 complex, leading to activation of the helper T cell, and the helper T cell will then release cytokines that will help to stimulate activation of other lymphocytes, including cytotoxic T cells. So cytotoxic T cells are T lymphocytes that have the CD8 positive T cell receptor complex, an infected cell that has an intracellular pathogen such as a virus, will display antigens from that intracellular pathogen using the MHC1 protein, the CD8 T cell receptor complex on the surface of a cytotoxic T cell will bind to the antigen MHC1 complex on the surface of the infected cell, this will lead to activation of the cytotoxic T cell, and the activated cytotoxic T cell releases perforins and granzymes, perforins will create pores in the plasma membrane of the infected cell, then granzymes enter through these pores, and the granzymes are proteolytic enzymes that will activate the apoptotic cell death mechanism. Whenever a naive T lymphocyte is activated by antigen presentation, that T lymphocyte will divide making more T lymphocytes that all have the same T cell receptor. Some of the T cells produced in this process will be memory T cells that are long lived in order to provide an immunological memory, and some of those cells will be the effector cytotoxic T cells that can work as part of the cell mediated immunity to activate apoptotic cell death in the cells of our body that have become infected with intracellular pathogen. So the mechanism where a lymphocyte becomes activated and divides to make more lymphocytes that all produce the same receptor is known as clonal selection, and this mechanism of clonal selection produces lots of lymphocytes that can all defend against the same infection, and this provides the immunological memory as well as making the adaptive immune response much more efficient as it can target a specific pathogen. In the cell mediated or cellular immunity, the effector cells are cytotoxic T cells that have the CD8 T cell receptor complex and are activated by antigen presented with an MHC1 protein by an infected cell of our body. When a cell of our body is infected with an intracellular pathogen such as a virus or an intracellular bacteria, the infected cell will display antigen from that pathogen using the MHC1 protein, and this will activate a cytotoxic T cell that has a T cell receptor CD8 complex that can bind to that antigen. Once the cytotoxic T cell is activated, it releases its perforins and granzymes leading to the controlled destruction of the infected cell through apoptosis. The apoptosis mechanism of programmed cell death is a controlled and efficient means of destroying and removing these infected cells without releasing the intracellular pathogens to infect neighboring cells.