This video, we're going to begin our lesson on natural killer cells. And so first, we need to recall from some of our previous lesson videos that natural killer cells are commonly abbreviated as NKs. And these natural killer cells are lymphocytes, just like the T cells and the B cells that we've been talking about so far. However, unlike the B cells and T cells that we've talked about so far, which are part of adaptive immunity, these natural killer cells are not considered part of adaptive immunity. And this is because these natural killer cells lack specificity in antigen recognition. And so these natural killer cells are considered part of innate immunity, and they're referred to as innate lymphoid cells. Now the role of these natural killer cells is to destroy infected host cells by recognizing irregular patterns on the host cell surface. Now these natural killer cells when they do recognize these irregular patterns on infected host cells, they release these "death packages" that include molecules such as perforin, which creates pores in the membrane of the infected host cell, and it also releases other protease-containing granules. And those proteases are basically enzymes that destroy proteins. And so collectively, these death packages released by the natural killer cells can induce apoptosis in the infected host cell. Now the natural killer cell can actually recognize one of two irregular patterns on the infected host cell.
The first irregular pattern that it can recognize is the lack of MHC class I on the surface of the infected host cell. And so the lack of MHC class I can actually be a result of some viruses. So some viruses have evolved complex mechanisms that allow them to, basically interfere with antigen presentation. And so when it does that, these infected cells will not have MHCs on their surface. And so these natural killer cells are able to respond to cells that lack MHC class I on their surface. Notice that in this image down below we're showing you how natural killer cells can induce apoptosis in cells that lack MHC molecules. And so notice over here on the left hand side, we're showing you a virus infecting the host cell. And this particular virus, again, has evolved mechanisms to interfere with antigen presentation. And so notice that whereas the original cell had MHC I on its surface, the infected cell no longer has MHC I. So the virus is preventing the cell from displaying MHC class I molecule. So notice it does not have those molecules on its surface anymore. Now, what this means is without the MHCs the T cells will not be able to respond. However, the natural killer cell is able to respond and essentially eliminate the cells that do not have MHC on their surface. So the natural killer cell, which is this pink cell over here on the right-hand side, comes along and it recognizes the lack of MHCs and it will release death packages that include the perforin to basically, create pores in the membrane and also protease-containing granules. And of course, the protease is going to include enzymes that break down proteins, and that is going to lead to the apoptosis of the infected host cell.
The second irregular pattern that, these natural killer cells can recognize on infected host cells is that these natural killer cells can respond to the Fc region or the constant region of IgG antibodies that are bound to the surface of an infected host cell. And that will lead to the processed antibody-dependent cellular cytotoxicity, or ADCC, which we got to talk about a little bit in some of our previous lesson videos. And so if we take a look at our image down below, notice that the natural killer cells can induce apoptosis via ADCC, antibody-dependent cellular cytotoxicity. And so over here on the far left-hand side, notice that we have a host cell, that is infected, and this host cell, that is infected is going to have specific proteins on its surface. And IgG antibodies can recognize and bind to those antigens on the target cell surface. And so these antibodies are somewhat marking this infected host cell. And so, the natural killer cell, the NK cell, can come along and recognize the Fc region of these antibodies. And the Fc region is the constant region, basically, the bottom of the Y shape of the antibody. And so the natural killer cell binds the Fc region of the IgG antibodies that are bound to the target cell. And when it does that, it's going to initiate the release of death packages that include the perforin and those protease-containing granules to ultimately induce apoptosis in the target host cell. And so ultimately what we're saying here is that these natural killer cells, they lack specificity. And so, they're part of innate immunity. However, they can work alongside adaptive immunity because again, it's recognizing the Fc region of the antibodies. And they also can kill target cells that lack MHC molecules. And so this here concludes our brief introduction to these natural killer cells, and we'll be able to get some practice applying these concepts as we move forward. So I'll see you all in our next video.
