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Video transcript
So we've talked a lot about different cells in the immune system and what they do and how they do it, but one thing we haven't talked much about is where they do it. So in this video, I'd like to talk a little bit about exactly where the different cells are acting and how they move around. So let's start with neutrophils. And neutrophils circulate in the blood. So we're going to draw a little cartoon of blood circulation here. So blood circulates in a closed loop. I'm leaving out the heart and I'm leaving out the lungs, all that. Let's just say blood is circulating round and round like this. So neutrophils circulate in the blood. So let's draw a couple of neutrophils there. They're actually the most common immune cell. So these are neutrophils, and in case you don't remember how to spell that, let's write it here, neutrophil. And neutrophils circulate round and round in the blood, but that's not actually where they do their work. They do their work only when there's infection. And that infection usually occurs out in the tissues. So let's say we've got an infection out here, a little bacterial infection. Now the neutrophils are needed, but they're needed out here. The question is how are they going to get there? And the answer is well they're going to have to squeeze out through the cells of the blood vessels to get to this tissue, but the follow-up question to that is how do they know to squeeze out here, and not to squeeze out here for example? And so it turns out that you have these cells out here called macrophages which I hope you've heard of by now, macrophage, let me write that out for you as well. And those macrophages are phagocytes, see the work phag in their names, you could probably get that. They're phagocytes, so they're going to gobble up some of these bacteria. I'll draw a couple of them in there and they're sort of going to go into action because they'll notice that there are bacteria there. And when they're in action, they're going to release some chemical signals. We don't really need to go into a conversation about what those are, but they're going to release some signals that will then tell the endothelial cells here, I'm going to draw a couple of them, that are going to alert the endothelial cells lining the blood vessels that there is an infection here in this tissue. And as a result, these endothelial cells are now going to express these little proteins here, this is very detailed, but it's just to give you a sense of sort of the flow of information here. They're going to express these little proteins here that are going to stick to those neutrophils. Those neutrophils are going to be coming by and they're going to get stuck on these little proteins that attract them. They'll be stuck there, and then because they're stuck there, they'll have the opportunity to squeeze through between these cells, and obviously other proteins and stuff are involved there, but that's a whole lot of detail that we don't really need right now. So neutrophils are going to get out here. And do you recall what the neutrophils are going to do? Well, the neutrophils are also phagocytes and so they're going to gobble up a bunch of these bacteria. They're going to gobble them up, chew them up, digest them, what not, and then once they've done that, they are just going to die. Because that is their job, is just to eat up some of the bacteria to get rid of it because you want to get rid of it. So they're just going to die and it actually happens pretty quickly. Their usual lifespan is only five days. And once they die, they become pus. So a little later that's what you see when you see pus. But really the important thing in this whole diagram is the process by which the neutrophils squeeze out between these endothelial cells, because usually there's only a very, very small hole. There are only very small holes between endothelial cells and certainly not big enough for a cell. So it's an active process by which the neutrophil kind of pries apart two endothelial cells to squeeze through and afterwards the endothelial cells come back together again to seal off the blood vessel. And it's worth noting that immune cells only ever go in this direction. They only cross from the blood into the tissue. And it's actually impossible for them to go from the tissue back into the blood. And so that raises a question. Because in previous videos, we said that the macrophages start here. They eat up some of these bacteria and then they want to go present these bacteria to B and T cells. They want to show these bacteria to actually really especially T cells, so that the T cells that are capable of fighting this infection can come fight it. But the B and T cells that are going to fight this are not located out here in the tissue. And so the macrophage needs to go to them and so then you might have the question well, does the macrophage come from this tissue into the blood and go find the B and T cells? And the answer as we've said is no. Because it's impossible for cells to go from the tissue back into circulation. And when I say impossible, I'm sure there are some exceptions, but for the most part, it doesn't happen. And so the answer to this conundrum is that these macrophages need to use a different mode of transport to get to the B and T cells. And I'm drawing that here. And this is a lymphatic vessel. And if you're totally unfamiliar with this, perhaps afterwards you should go watch some lymphatic system videos. But lymphatic vessels have little holes in them that actually make it fairly easy for cells and proteins and other things to get in. So the macrophage is going to get into lymphatic vessels and that's going to allow it to travel to the nearest lymph node. And so here we've got a lymph node and it has some T cells. T cells right there. And some B cells as well. And it's there that the macrophage can do it's very important action, which we'll mention here because it's important. I'm going to draw a little star there. The star is antigen presentation. Antigen presentation. And there are other videos on that, so we won't really go into that here. But it's I think useful to know that that's where it happens. And as a result of antigen presentation, hopefully we have some B and T cells that are specialized to fight this bacterial invader. Hopefully we've found them and now they can go join the fight. So what's the obvious question? It's how are they going to join the fight? How are these guys going to get there? They do not travel backwards through the lymphatic vessel because lymphatic flow is one directional. It flows this way. But luckily lymphatic vessels all end up draining into the blood and this is our cartoon, but if this weren't a cartoon, this end lymphatic vessel where the lymph fluid is going back into the blood, would be called the thoracic duct. I'm just mentioning that for orientation. So luckily all the lymph in the body, the vast majority actually goes directly through the thoracic duct which is this fat final lymphatic vessel that drains into one of the big veins in your body. But really all you need to know is all lymph eventually drains into blood and so that's exactly the method that these T cells can use to get back into the blood. And now once they're in the blood, what are they going to do? Well they're going to do essentially the same thing as these neutrophils. And maybe the chemicals, the proteins are slightly different, but essentially it's the same process. They'll come here. They'll realize that this is where they need to go because of certain chemical signals, and they they'll squeeze through between the endothelial cells and get here. And I'll try to draw a T cell out here, even though there's not much space left, so that you can see the points. And note that we also have B cells. And you might be wondering, are the B cells going to go out and do the same thing? And the answer is that some of them might, but B cells don't really need to be out here. Because what B cells are going to do, is release antibodies, and here are some antibodies. They're going to release antibodies which can actually just float around the body themselves. And I'll draw a couple around the blood. And some of them will get out here to the tissues where they need to do their work. So B cells for the most part can kind of chill out in the lymph node here, which I hope I mentioned that this is called the lymph node. If not, let me write that. They're hanging out in the lymph node there, pumping out these little antibodies, who can then go travel and join the fight themselves.