So with this folder we're going to get into talking about cell mediated immunity. With the last folder we got into the other branch of adaptive immunity, we looked at humoral immunity and you learned that humoral immunity responds to pathogens that are in body fluids or what we use to term body humors. These are things like the blood and the lymph. We've also talked about since we've been in the immune system, the fact that certain pathogens, so things like viruses, some bacteria are able to actually insert themselves into cells of the body and once they're down in those cells, they're actually protected by the cell membrane or the plasma membrane itself from the humeral immune system. Because of that, we've gotta have this second branch of adaptive immunity, which goes after harmful substances that are hiding out in your own body cells. And this is where cell mediated immunity comes in. So when we were looking at humeral immunity, I mentioned that kind of the major cell type that kind of gets things going in humeral immunity are the B cells or the B lymphocytes. And with cell mediated immunity we see that role being played by T cells or T lymphocytes. So those are the white blood cells, the lymphocytes that are going to complete their maturation within the thymus. We also looked at a diagram when we were talking about humeral immunity that showed a B cell binding. It's one specific antigen and I mentioned that once AB cell has done that, it acts as an antigen presenting cell or an APC. It goes and finds a helper T-cell and that helper T-cell is going to activate the B cells so that humoral immunity can get going. So since we're talking about helper T-cells, I want to discuss a little bit and kind of show you how that activation of B cells occurs through a helper T cell. So if you look at this diagram over here, we have in this purple cell on top, um, AB cell that has bound to its one specific antigen. So here is the MHC two receptor and we've got the little antigen that it's bound to here represented in red. Below that we have a helper T cell. So this B cell is currently acting as an antigen presenting cell. It's found something in the body that it believes to be harmful and it's showing it to helper T. I want you to notice that the helper T is releasing some chemicals back onto the B cell. So that's what these little kind of yellow circles in here represent are chemicals that are being secreted by the helper T cell back onto the B cell to activate that B cell. Once the B cell's activated, that's when it's capable of dividing into clones and from there into plasma cells and then into antibodies. So help for T cells, which are a part of the cell-mediated system. Are kind of the generals, um, of the entire immune system if you will. They are going to bind to those things that are displayed by antigen presenting cells and make the final determination as to whether or not they're looking at something that's actually harmful. If they are their going to activate that B cell so that it will start to divide and eventually we'll get antibodies being produced. So they're kind of um, in charge of activating the humeral system. I wanna go back a little bit to the cell mediated system and then look at how these helper T cells, which are kind of our major player in the cell mediated system, are involved in activating other T cells that go after cells that are harboring something that's foreign such as a virus. So to do that, I'm gonna go to this last slide and we're going to look at how helper T cells, which again are our generals work to activate another type of T cell, which is known as a cytotoxic T cell or a TC cell. Cytotoxic T cells are kind of the ground troops, if you will, of the cell mediated system. They are actually going to go after those cells that are harboring a virus or something else that's foreign and harmful and work to destroy those cells through lysis. So the process of activating a cytotoxic T-cell has two steps to it and it's demonstrated for you in this diagram over here. Here we've got our helper T-cell. Once again, this is an antigen presenting cell. So in this particular diagram it says we're using a dendritic cell as an antigen presenting cell. It could be AB cell as well because they act as antigen presenting cells as do macrophages, those phagocytic cells. But I want you to notice that the first thing that happens to activate a cytotoxic T cell, these ground troops in the cell mediated system is we first have to activate the helper T. So here we've got a helper T that is being displayed in antigen that was found by this dendritic cell, the antigen presenting cell, and we're getting activation of the helper T through that process. Once this helper T cell has become activated, the second step occurs and that step is known as uh, co-stimulation. During co-stimulation, the activated helper T cell causes our APC, our antigen presenting cell, in this case the dendritic cell to sprout a surface receptor that wasn't there previously. So that's this receptor right here. This receptor only sprouts on this APC after it has bound to helper T and the helper T has become activated. This receptor that we have right here provides a place for cytotoxic T cells to bind and the binding of these cytotoxic T cells. To this receptor is going to activate the cytotoxic T cells so that they go out into the body and they start looking for cells of the body that are displaying that particular antigen. And when they find those cells, they're going to destroy them through lysis, which is a process where basically they puncture the cell membrane and that causes all of the internal contents of the cell to spill out, which destroys the cell.