 When this video we're going to talk about what's called immunological surveillance and the cell that's responsible for immunological surveillance is the NK or natural killer cell. It's a type of lymphocyte. So we won't talk about it again with the T cells and the B cells. It's a separate type of lymphocyte. And it is involved, what it does is it bounces around and it's looking for cells that are either cancerous or infected with viruses. And what happens is if it bumps into a normal cell, then it recognizes what they're called MHC1 or major histocompatibility complexes that say this cell's normal, nothing wrong with it, nothing to see here, I'm going to move on. When it bumps into a cell that's either infected with a virus or maybe an intracellular bacterial pathogen or a cancerous cell, it's going to see these distress signals on the surface. And mainly it's by what they get rid of. They get rid of these MHC complexes. So the natural killer cell doesn't know what's wrong with this cell. That's why it's part of the non-specific defenses. It just knows that there is something wrong with this cell. And what it's going to do when it bumps into this infected cell, like here you see an example of a virus infected cell, it is going to use chemicals like perforins that are going to direct this cell to kill itself. Remember, that's called apoptosis. So the job of the natural killer cell is to ask these cells to kill themselves. And the reason it does that, it's kind of interesting, that it directs apoptosis rather than killing this cell directly. If it were just to lice this cell and rip this cell apart, it would actually release its payload, potentially releasing these bacteria or viruses. By asking this cell to kill itself, it's trying to stop the spread of the pathogen from going any further. So here at the bottom, it has deposited its payload, primarily these perforin chemicals that's going to cause this cell to kill itself while it reloads so it can go and do it again and again. So that is immunological surveillance, the primary function of the NK, or natural killer cell. I hope this helps. Have a wonderful day. Be blessed.