白血细胞在身体中如何移动

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 gonna draw a little cartoon of blood circulation here so blood circulates in a closed loop I'm leaving out the heart 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 were circulate round and round the blood but that's not actually where they 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 gonna 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 you have the word phage and their names you could probably guess that they're phagocytes so they're gonna gobble up some of these bacteria we'll draw a couple them in there there and they're sort of gonna 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 need to really go into conversation about what what those are but they're going to release some signals that will then tell the endothelial cells here I'm gonna 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 it's just to give you a sense of sort of the flow of information here they're gonna express these little proteins here that are gonna stick to those neutrophils so this neutrophils are gonna be coming by and they're gonna 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 you know 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 gonna get out here and do you recall what the neutrophils are gonna do well the neutrophils are also phagocytes and so they're going to gobble up a bunch of these bacteria they're gonna gobble them up chew them up and digest them whatnot and then once they've done that they're 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 gonna die and it actually happens pretty quickly their usual lifespan is only five days and once they die they become fuss so a little later that's what you see when you see pus but really the important thing in this whole diagram is this 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 it's cell so it's an active process by which the neutrophil kind of cries apart to 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 to actually really especially T cells so that the 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 just 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 you know 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 you know proteins and other things to get in so the macrophage is going to get into the lymphatic vessel 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 well 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 V and T cells that are specialized to fight this bacterial invader hopefully we found them and now they can go join the fight so what's the obvious question it's well how are they going to join the fight how are these guys going to get there they do not travel backwards Google emphatic 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 were in a cartoon this ends a lymphatic vessel where the the lymph fluid is going back into the blood would be called the thoracic of conduct 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 you know there's a 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 gonna do well they're gonna do essentially the same thing as these neutrophils and you know 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 then 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 be cells and you might be wondering are the B cells gonna 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 gonna do is release antibodies so 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 the lymph node here which I hope I mentioned that this is called the lymph node if not when you write that they're hanging out in the lymph node they're pumping out these little antibodies who can then go travel and join the fight themselves