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吞噬细胞在先天或非特异性免疫中的角色。中性粒细胞、巨噬细胞和树突状细胞。II类主要组织相容性复合体(MHC II)。 Sal Khan 创建




the whole point of the immune system immune system is to keep out shady things from your body or if they get in to kill them so that those shady things would include shady proteins that can do damage to your body viruses viruses bacteria even eukaryotic parasites parasites and an even fungi so all sorts of things that if they you if they were to enter your body they would cause some form of disease these are collectively called pathogens pathogens so the whole point of the immune system is on a first line of defense keep these things out and then if they were to get into your body to kill and eliminate them from our system so that we don't get sick and so that we don't die so I already just mentioned that there's kind of two lines of defense and even with those there is kind of sub classifications the first line of defense I'll just called it the first line which is essentially just to keep things out keep keep all of these pathogens out keep out and there's some obvious ones there's our skin our skin keeps pathogens out and actually even our our the oils on our skin are a little bit more acidic and it's hard for some types of bacteria to to thrive in that type of an environment you have our your mucous membranes mucous membranes and in the mucus there are some there's some chemicals that maybe make it a little bit more difficult for bacteria to survive and then you even have acidic environments like your like your stomach acid I'll keep that in the same color stomach acid you might not view the stomach your stomach as the outside of you but it fundamentally is your whole digestive tract which I'll make videos on in the near future is really on the outside of you you can you can simply model really most well most vertebrates bodies it's kind of a doughnut and the our digestive tract is kind of the inside of the doughnut so stomach acid is on the outside of our our real bodies and you can imagine that's a hard environment for a lot of these pathogens to survive in so that's the first line of defense but we know that that's not good enough that sometimes these things can get into our bodies and there we have to start thinking about the second line of defense the second what do we do once things are actually in our body second let me switch colors so then you have your second line of defense second line and here in both the first and second line I'm talking about nonspecific immunity and this is going to make a lot of sense when we start talking about specific immunity so both of these are non nonspecific and when I say nonspecific or you can also call them innate it means that they just generally respond to things that appear bad they don't remember the bad things that came before they don't respond to a particular type of virus or a particularly well they do respond to every type of virus or every type of bacteria but they don't say oh this is virus type you know ABC or this is bacteria type ABC they just say this is a virus let me get rid of it or let me not let it in this is a bacteria let me get rid of it or - let me not let it in it doesn't say it doesn't know what type of bacteria it's dealing with so this is all the nonspecific or an8 immune system and we'll do I'll go into a lot of detail onto the specific immune system because you can imagine it becomes very complicated or interesting when you start thinking about your body somehow remembering a virus that it's seen before and being able to respond better to that virus or that bacteria or that protein the second time it sees it so we're dealing with nonspecific in this case and the the second line of your nonspecific immunity there are two things one is an inflammatory response inflammatory matauri response response and I'm gonna do a whole video on this but you in general we've all experienced inflammatory responses when you when you see blood flowing to a certain part of an area and you see there's pus and there's I'm gonna go into a lot more detail what an inflammatory response actually is but that's one of your and what it really is doing is bringing blood and bringing cells that can fight whatever type of infection you have there it's bringing them to the site where that you know maybe you got a cut or maybe where a lot of the bacteria or whatever the pathogen is so inflammatory response is all about bringing fluid and fighters to the fight so bringing stuff to the fight let me write that and I'm gonna do a whole video on that bringing stuff to the fight you can almost met bringing weapons to the fight to the fight but the the byproduct is is that part of your tissue or that part of your body gets inflamed a lot of fluid there a lot of I guess byproducts of the battle that goes on there and we'll do a whole video on that and then the other second line of defense and it's actually part of the inflammatory response or phagocytosis or phagocytes phagocytes and really what I want to do over the rest of this video is talk in a little bit more detail about phagocytes because once we understand what phagocytes do that's a pretty good building block for under four going into the specific immune system and actually it'll help lead into the discussion on the inflammatory response as well because phagocytes are really part of the inflammatory response so phagocytes are just a class of cell that can eat up pathogens they can eat up other things really but when we talk about the immune system we're talking about pathogens so let's say that this is a phagocyte right here this is a phagocyte right there you know it has some kind of a nucleus whatever I don't have to focus on the inside of the phagocyte it's a it's a traditional eukaryotic cell but what I want to do is see what happens when a phagocyte encounters a a foreign particle or a foreign bacteria so let me say this is a foreign bacteria right here so the phagocyte we've already said is nonspecific what it does is it has receptors that respond to just things that it knows are bad you could imagine these are super sensors you know so maybe these are super sensors right here these are super sensors that maybe these are super sensors for bacteria and the bacteria have proteins on their surface that maybe look something like that obviously they don't look exactly like that I'm just drawing them as kind of a why in a triangle so you can see that they fit but once these two guys connect once these two guys connect so let me draw the situation where they have connected so this is the bacteria this is the pathogen and it's really the same idea with the virus or any other type of thing and we'll actually see in future videos that these guys can actually be tagged by other molecules which makes these phagocytes want to attack them even more but let's say once they're bonded so you know that's my the bacteria the invading pathogen and now it is bonded it has triggered the receptor on this phagocyte this phagocyte will start to engulf it'll wrap around it'll wrap around this pathogen so it'll start wrapping around the pathogen like that it'll start wrapping around the pathogen like that and these two ends are gonna eventually are eventually going to meet they're eventually going to meet and I could still draw let me draw the nucleus in a different color so we know it's not really related to the phagocytosis so maybe that's the nucleus and there's other organelles sitting in here but then once these two meet what's what's it gonna look like then all of a sudden all of a sudden that bacteria is going to be completely engulfed it's going to be inside of the cell so now the cell once these two ends meet and these membranes merge then this guy is going to be in his own little membrane bubble or you can almost imagine it's in its own little vesicle so this is the pathogen the bacteria in this case but the phagocytosis the process is completely identical in terms of how it engulfs things if it was a virus or some type of other foreign protein or any type of really foreign molecule let me and actually sometimes it doesn't even occur to foreign stuff it can occur to stuff that is part you know dying molecules that are not foreign that just need to be cleared out but we'll just focus on the immune system on foreign things right now so was merged let me draw so then this membrane right here will completely merge and go around this guy like this and of course you had your receptors and you know who knows if they're still there by the time and let's just draw them there so you see that that part is that part but once it's fully engulfed this thing is called a phagosome this is a phagosome phagosome which is really just a vesicle that contains that foreign particle that you want to get rid of and then other other other fluid or vesicles that contain that contain let me drew in a different color that contain things that can eat up that can eat up this phagosome so let's say that you know here right this is some vesicle that contains things lysozymes and it contains really reactive species of oxygen so let's say that you know it contains a bunch of stuff here and if this comes in contact with really almost any biological compound it's gonna do some damage but once this once the once the the the pathogen is completely merged inside the cell this little package will merge over here and it will dump its contents into this phagosome into this vesicle containing the pathogen and then break it up and then break it up it's essentially digesting it it's I essentially digesting it so obviously the first role is it just got it out of the way and it killed it and then the second role and I'm just gonna give a little tidbit right here we're gonna do it in a lot more detail in future videos it breaks it up so let me draw the whole thing now I want to keep my colors consistent it'll break it up so now the thing is all broken up it's all broken up this is the membrane inside so that thing is broken up into constituent proteins and other molecules and then what what the phagosome or what the phagocyte does what the phagocyte does it'll actually take some subset of these molecules some subset of these molecules some subset of the proteins it'll break them now proteins are just sequences of amino it's only when people say proteins they're talking about long sequences of amino acids when people talk about short sequences of amino acids or a protein that's broken up a lot they talk they refer to it as a peptide chain a peptide chain is a shorter chain of amino acids so this guy will take some special peptide chains some special pieces from the thing it just killed attach them attach them to some other proteins so it'll take maybe a little piece of this of this of this bacteria right now attach it to other protein let me just do that in pink it'll attach it to this other protein which is called a major histocompatibility complex and if we're talking about phagocytes this will be a major histocompatibility complex type - and that's that's a mouthful of words so let me play it so this is a and this is it sounds very a strange word but we're gonna see this a lot so this is major histocompatibility had a bility complex complex so they abbreviated MHC this is a protein and it bonds with this peptide that was kind of chunked off or digested off of this invading pathogen and then the this-this-this phagocyte will then present it onto its membrane so this combination the complex of the MHC and this in this case it's going to be an MHC 2 protein we're going to talk so MHC type 2 we're going to talk about type 1 in the future it's going to take this complex and then present it on its surface and then it's going to present the combination of the MHC type 2 and and the little peptide chain that came from thing and presented on its surface and the reason why I'm going through all of this a pain of explaining this process you're like hey we already got got rid of the thing and killed it why is Sal worried about what we do with the peptides this is crucial to our immune system because we'll see other specific parts of our immune system remember so far everything is nonspecific this guy just said oh this is an invader it doesn't know the type of an invader it just says hey let me bond to this thing and kill it it's not it's it's it's it's one of these things that I know are foreign to my body so it kills it but now it can leave it on its surface and now the specific parts the parts that actually have memory and attack specific things can say gee mister phagocyte look you've killed something let me see if I if I have some specific reactions that can be triggered by this thing that you're presenting so many phagocytes are also called antigen presenting cells antigen presenting cells and I'm gonna go into more detail on what exactly an antigen is you know I called this thing a pathogen an antigen is essentially you can view it as a protein or a a peptide chain that will trigger or that can be dealt with within the immune system I'll be a little bit more the specific immune system and I'm gonna be a little bit more nuanced about it when I talk I'll make a whole video on antigens and antibodies but right now you can just view it as you know a peptide chain right there an antigen is just a protein or a part of a protein so this is presenting an antigen on its surface that can later be used by other parts now the one thing that you know there there are many many types of phagocytes and just to give you you know just so when you see different words you don't get confused by the different types of phagocytes I'll do a little review of those right now you have neutrophils neutrophils these are actually the most common of the phagocytes and these are kind of the fast and numerous responders so these get to a location of infection very fast so fast and abundant and you know phagocytes don't necessarily just have to kill in this way I mean they're called phagocytes because they engulf this way but will in future videos talk about other ways that they can release chemicals or even DNA nets to ensnare pathogens but neutrophils are fast and abundant and then you have macrophages which are kind of do the on some level they're the most versatile and do the heavy lifting but they're all so phagocytes and then you have dendritic cells and when you first see the word dendritic cell you think hey does this somehow relate to dendrites of the nervous system and no they have nothing nothing to do with the nervous system nothing nervous system about them nothing nervous the reason why they're called dendritic cells because they look like they have dendrites they'll look like you know they have these little things coming off of them just like that and if you kind of just view it as a division so they look like neurons on some level but they don't participate on the in the nervous system at all and these tend to be the best activators of the specific immune system that we'll talk about in future videos so I'll leave you there and we'll talk more about all of this in the next few videos