克隆选择

in this video I'd like to give an overview of how the B and T cells in your body are prepared to fight infections and so let's start with where B cells come from so where do B cells come from they come from the bone marrow so here's a bone and inside it we have some bone marrow and it's from there that B cells come from but the process by which the B cells are prepared in the bone marrow is actually very interesting and unique in terms of the human body because well like many other places in the body the B cells divide and form millions of descendants and that sort of is how your whole body grows and comes to be as big as it is but for B cells there's something very different from other parts of your body and that is that each daughter cell that grows up in the bone marrow is different from its parent and it's also different from all of its siblings so just to remind you B cells have these receptors that later on will be their antibodies but each daughter cell of this original B so we'll have a different receptor and that's because in the process of being made these daughter b-cells will have a slight shuffling a slight changing in their DNA but specifically for this receptor and so by the time they're created they have a unique receptor that will identify a unique criminal a unique bacterium or virus or something in the body but that receptor will be useless for all the other criminals in the body so it's very specific but very powerful if it finds the thing that it wants to kill so this is happening in the bone marrow and basically the same exact thing is happening for t-cells except it's not happening in the bone marrow that's happening here you know what this is called this is called the thymus and I'm drawing it a little bit bigger than I should maybe but that way you can see it clearly the thymus has two lobes or two parts and it's located right behind your sternum so t-cells come from the thymus and very similar to b-cells they have a unique receptor it's not quite the same and it doesn't become an antibody later on they have a unique receptor and while these t-cells are dividing and creating millions of daughter t-cells each of these daughter t-cells will have its own unique receptor that will also identify a unique criminal or bacterium in the human body and we remember that these receptors actually bind antigens so pieces of those criminals that are presented to them by antigen presenting cells and we'll revisit that in a second so all this is occurring in the thymus and so this is step one of the process and it's unique to the human body because we end up with all these actually literally genetically different B and T cells that are very specific to things which may or may not even exist out there because they're really created at random now the next thing that happens is that these b-cells and t-cells once they're ready are gonna migrate to a lymph node and you have lymph nodes all over your body about 600 of them but let's say that they go to one here in your armpit they're gonna migrate to that lymph node for example and that is where they're going to wait for their their special criminal that they can react to do so they're gonna go over there to that lymph node and let's draw that down here maybe significantly bigger so here's the lymph node and you're gonna have your B and T cells all the different kinds of them lying in wait there and they have their special receptors each one is specific to something else so these guys have migrated here to all the different lymph nodes of the body and now these guys are ready and so the next step is really infection so let's look at a tissue in your body with some cells these are cells let's say it's over there so it's not too far from that lymph node there and let's say it gets infected so here are some bacteria which are causing damage and one of the things we have out here is these cells which are part of the immune system which are called dendritic cells dendritic cells and these cells can eat some of these bacteria so they can actually kill some of those bacteria they can ingest them but it's actually not quite to the point that they can defeat this infection and that's not really their purpose and their purpose is actually more to present these bacteria to the B and T cells that we've been talking about and so the way that they do that is that they come here into these lymphatic vessels which start here in the tissue and they go up to the lymph node where they're gonna present these bacteria to these cells and at the same time some of the bacteria might also get swept in there and might go into the lymph node and so you get some of them out here you get some bacteria and maybe just some pieces of bacteria that got broken down and you also have your dendritic cells here which are antigen presenting cells which means that they have this protein that we've talked about called MHC MHC on which they present pieces of those bacteria that they have ingested now these T cells here are unable to identify these bacteria when they're just freely floating they need this antigen presenting cell to show this piece of bacteria to them and only the T cells which can recognize this very specific pathogen are going to see it and bind to it and react to it so in this case it's this guy he's gonna bind to that little piece and recognize that his invader the one guy that he's been created to kill is somewhere in the body meanwhile these B cells are made to react directly to these bacteria without this antigen presenting cell and so they are the specific one that is specific to this bacterium is gonna see it bind to it and also be alerted however all these other guys they're not going to know that anything's going on because they can't bind to enemies any of these pathogens that were brought into the lymph node and so this right here is step two when the infection occurs and the right B and T cells are alerted and what are they gonna do now well now they're going to activate they're going to jump into action because finally they're needed and now these B and T cells are going to start replicating like crazy so here are the two the two lucky winners who got selected and they are gonna say well now that we know that we are useful for fighting this infection we better replicate like crazy because right now there's only two of us and that's not nearly enough so they're gonna start replicating and each of their descendants unlike this kind of replication now each of their descendants is going to have the same receptor it's gonna have the same receptor if it had different receptor this whole process would be useless because it wouldn't be able to fight the specific infection that we've just identified and I should mention that not all these guys are gonna do the same thing so maybe most of them will become effector cells which means that they're gonna be ready to fight this infection right now but some of them are gonna become memory cells which means that they're not going to actually fight right now they're gonna wait until the next time that this pathogen comes around and the reason why we create these guys because if we get this bacteria once there's a high chance that we're gonna get it again because really you create billions and billions of these different receptors but only a very small subset of them will actually end up being pathogens that you see and so this is step three of our process which has finally resulted in an army of B and T cells ready to fight this specific invader and so that's what they're gonna do now the B cells are gonna pump out antibodies to attack this bacterium and the t-cells some of them are gonna stay in the lymph node and some of them are gonna travel out to this site to join the battle and so this whole process can actually be called clonal selection clonal selection which is a name for basically for the theory of this process when it first came out so this is clonal selection the reason it's called clonal selection is because you're gonna select the B and T cells that you need and then you're gonna start cloning them like crazy into this army and the process involves all these these very unique specific B and T cells coming from the bone marrow and famous to the lymph node where they're gonna be met by these bacteria that are infecting your skin that are going to travel through the lymph node through the lymphatic vessels to the lymph node and so they'll meet there and then the armies will be prepared