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Current time:0:00Total duration:11:33

Professional antigen presenting cells (APC) and MHC II complexes

Video transcript

in the very first immunology video where we talked about nonspecific defense mechanisms we said if we had some type of a pathogen let's say it's a bacteria that our phagocytes can recognize either proteins on the bacteria or maybe if it's some other type of pathogen some other marker on the pathogen doesn't know what type of pathogen it is but that's enough for the phagocyte and it will engulf it'll engulf the pathogen so let's say this is a phagocyte it you know somehow one of their receptors touches one of the proteins that's on the surface of this of this bacteria in this case and says gee that's not a good thing to have around and so it'll golf it so the membrane is just going to surround is going to surround the pathogen end of it and this part is going to go in it's going to kind of pinch in and the pathogen is going to get surrounded and get engulfed phagocytosed by the phagocyte so the end product we saw this on the first video where I talked about phagocytes is we're going to end up so then the memory is going to be completely around this is going to pinch in and bubble around this thing and so we're going to have the pathogen sitting inside of the phagocyte it's going to be surrounded by a membrane that membrane is called a phagosome and we learned there's different types of phagocytes could be a macrophage macrophage could be a neutrophil could be a dendritic cell dendritic dangered Excel and we have it like this and then we also saw in that video that's not just done the macrophage doesn't just digest this thing that you know that by itself is very useful it got this bacteria out of the way if this was a virus it got that virus out of the way but does even more than that it takes that and then it Lices it or it breaks it up it doesn't have to actually use doesn't have to just oh it breaks it up in there are several ways it can do it but the bottom line is is it processes it and let me let me draw that it'll break up the pathogen and we saw in that first video on phagocytes and that will actually have a lysosome bond to it and dump all sorts of particles that are going to break up and cut up this bacteria in this case into its constituent molecules maybe you can draw that so it's going to come here and it's going to dump all of its contents and break it all up and then the leftovers some subset of the leftovers particular chains of the peptides and remember proteins are long chains of amino acids polypeptides are short chains so you take short chains of those and they're going to bond to special proteins they're going to bond to special proteins and this is essentially the topic of this video so let's say they bond to that special protein bond to that special protein and then those proteins get transported or get to the membrane or the outer surface of the cell and they present themselves along with the piece of the pathogen so the end product after we've phagocytosed this pathogen is that we have the phagocyte will look something like this and it will have these it will have these antigen presenting proteins I guess we can call them that had bound to parts of that original that original pathogen so let me do it right here so this right here so it has a little bit of the original pathogen on it that I do in green right here and these proteins right here these are called and it's a fancy word and I talked about in the previous video these are called major histocompatibility histo compatibility histocompatibility complexes complexes or Emmet's ma MHC MHC for short MHCs and when we're talking about a phagocytes or you know macrophages or dendritic cells that are particular cases of phagocytes the major histocompatibility complexes that they present after they digested this molecule this is an MHC class 2 so let me write this down this is an M H C class 2 protein it might seem like I'm really going into the minutiae of what these proteins are but we're going to see this is key for activating other parts of the immune system especially the cell-mediated parts of the immune system so this was the case with a macrophage or a dendrite they engulf something they chew it all up and then the parts of the chewed up thing that they ate they attach to these MHC 2 proteins and then these MHC proteins are go to the top surface of the cell the same thing or actually not that's quite the same thing a very similar thing happens with b-cells very similar so if we have a b-cell let me that's a good color B for blue but we know B stands for bursa but it could stand for bone marrow just as well let's say we have a b-cell it's got its membrane bound antibody on it and remember it's very specific to that b-cell and so all of the membrane bound antibodies all 10,000 or so of them on this b-cell they all express Express the same variable part so this is a particular b-cell so remember this was nonspecific when we talked about phagocytosis these guys just said hey you're a bacteria you're a virus I don't know what kind you are I'm just going to eat you up you look shady I'm going to eat you up I don't know what type you are whether I've seen you before we're talking about b-cells we're talking about the adaptive or the specific immune system and so these the variable ends of these membrane bound antibodies are specific to certain parts of certain pathogens to spread certain epitopes remember epitopes were the parts of certain pathogens that these specific chains can recognize and bond to so let's say that we're dealing with a virus in this situation and let's say the virus just happens to bond to this b-cell remember there might be other in fact there definitely are tons of other B cells around tons of other b-cells around but they're variable portions the variable portions on all of these other b-cells and actually let me do them in different colors so that the variable portions are all different and that's what I always find amazing about b-cells is that even though they come from the same genetic line they have their genes have been shuffled around and they're in their development so that they can produce billions of combinations of these proteins but or the variable ends of their ant so let's say we have some virus so let's say it's less let me say it's a bacteria when I did the other B cell example I said we're dealing with viruses but let's say some bacteria some new bacteria and it's and just some part of its surface it just happens to bind to only this B cell because this P cell has just the right combination so some part of his surface binds just to that B cell right there and that part of the surface that binds remember that was called the epitope that's that prototype part of the of the of the pathogen that binds to our variable sequencing it won't bind to this me seller this B cell because they have different sequences here but it binds to this B cell and then that starts the activation process we're going to sometimes this can this by itself can get the B cell activated but you normally need help from helper T cells and we'll talk more about that and we said once this happens once you get activated or the activation process starts this guy actually gets engulfed and I didn't talk about that in the last video just cuz I didn't want to go into too much detail so this this whole thing gets engulfed by the B cell by the B cell and then when it gets activated it proliferates it proliferates itself and you normally need the T cells there and some part of them become plasma B cells some part of them become memory B cells so these are memory memory and then you have the plasma B cells and remember the plasma B cells just say gee I've been activated I'm just going to produce a ton of these antibodies so the plasma B cell will just produce tons of these antibodies and start spitting them out so that they can attach to more and more of that pathogen and just mess them up in different ways either tag them the greens the pathogen tag them so that other macrophages or our phagocytes eat them up or tag two of them so you bundle them up so that they can't operate properly whatever I'm not going to go into detail on that that's when it gets activated but the interesting thing is is that the B cell will also do what the phagocytes do the B cell will also take this guy in will also take this guy in into the cell into the cell you know maybe he's initially attached to the antibody and break him up break em up take pieces of this take pieces of this pathogen and attach it to MHC 2 proteins and then present them on the surface so a B cell will also present the antigen will also present the antigen so this is also an MHC 2 complex a major histocompatibility complex and just so you know histo means tissue so this is related to whether something is compatible with the tissue in your body and we'll talk more about that and how it relates to transplants and all of that so this is also an MHC class 2c in in both cases whether we're talking about B cells that's recognize a very specific pathogen that and it could be a specific virus a specific protein a specific bacteria or in the cases of phagocytes that just say oh you look Shady let me take you in I don't know what type of bacterial virus or protein you are but in either case they both engulf them take pieces of them cut them up and present them on their surface in a complex with the major histocompatibility complexes so vie so cells that do this are called professional antigen presenting cells professional this is what the this is what they do for a living although they do other things as we've seen this guy the the phagocytes eat things the B cells generate antibodies or memories so that they can later be activated to generate antibodies but these are called professional antigen presenting cell and the antigen in question is this little piece of of the actual of the actual of the actual thing that you're trying to track that little piece that we cut the little actual part of the pathogen that's what the antigen so it's presenting the antigen it's professional because it does this it takes antigens from or it takes it takes pathogens from outside of our of our or in the fluid of our system and then engulfs them breaks them up and then presents them now there are also non professional antigen presenting cells and in fact most cells are in fact even these guys so actually I'm going to wait for the next video I realize all of my videos are getting along so you're probably thinking gee what are these okay so what these guys in in either case engulf them cut them up present them what is it good for you'll see that these are these these MHC 2 these are what are recognized by helper t-cells so it's it's a tall form part of the puzzle of how our immune system works in the next video I'll talk about MHC 1 presenting cells which is pretty much all body cells
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