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So let's talk about exactly how flu causes so much damage to ourselves and why it makes us feel so lousy whenever we get the flu. I'm going to start out by drawing the flu virus here. This is our influenza virus. And we have on influenza a couple of features we have to remember. So on the outside there's this little envelope, and what's on the inside of this envelope are eight bits of RNA. Eight pieces of RNA. And so this RNA is important to remember, because in the human cell, in our cells-- I'm going to draw one of our cells right here-- we have, instead of RNA, we have DNA. Remember. And so this is our nucleus, and on the inside of our nucleus is our DNA. So this is our DNA over here. So the virus has RNA, and we have DNA. And the outside of the human cell-- actually let me label this over here. This is human cell. The outside of the human cell has something called sialic acid. They're these little strands over here that are coming off. I'm drawing them far larger than they are in real life. They're not nearly this big, but they're these little tiny little things called sialic acid. And this sialic acid becomes very important in understanding how the influenza virus gets into and out of our cells. So on the outside, remember, of the influenza virus, there were a couple of proteins. And I'm going to draw one of these proteins here, and I'm going to make it look like a hand. So this is a little hand, and this protein is called hemagglutinin. In fact, previously I had called it the H protein, and you can call it that if you want. But the full name is hemagglutinin. And what hemagglutinin does is that it actually holds onto sialic acid. In fact, that's an easy way to remember it, right? Because H and H go together. It holds sialic acid. And that becomes very important, because that allows it the first step towards getting into the cell. Now there's another protein on the outside here-- I'm going to make it look like a pair of scissors, because that will kind of remind us what this one does. And this is called neuraminidase. And I'm going to-- neuraminidase. And I'm going to pass on explaining what it does, just for the moment. I'll tell you in a little bit what it does. So then the first step to get into the cell is for hemagglutinin to hold on to sialic acid. And then there are a few other small molecular steps that happen, important ones. But I'm going to suffice to say it gets inside. And once the influenza virus gets inside, these RNA segments, they are let loose. So these segments are going to start making their way towards the nucleus. And so once they get into the nucleus, they're in that same kind of area that the DNA is, and what they do is remarkable. They basically, they take over. These little RNA start making many copies of themselves. And what they want to do is make our human cell into a factory. They want to make a factory. And this factory is going to make little proteins, viral proteins, and it's going to make viral RNA. And what it's not going to do, the one thing that the cell is no longer going to be very good at doing, is its normal job. So the human cell, of course, had some job to do. And it's not going to have the resources or the time to do it, because it's basically being taken over by this viral RNA. So what happens then is that the viral RNA is basically turned it into a factory. And what it wants to do is make more and more copies of itself. So let me actually just show you what that would look like. Here's a daughter cell. Let's say it goes over here. And I'm going to clean this up a little bit, just to make sure that we're looking at a nice, neat picture. Let's say something like this. So this is our daughter cell in the other side, and these cells are going to try to make their way out of the human cell, right? Because now they're packaged, they're ready to go, and where do you suppose they want to go next? Where they're going to want to find their own human cell to invade, because they want to continue this process. So we've got more human cells over here down below. And we've got maybe, let's say, one human cell up here. So we've got new targets for this virus, and this virus is going to seek out these targets and try to make its way inside again using its hemagglutinin. But before it can do that, it's got to break loose, right? Because it's still attached to that sialic acid. And so here's where neuraminidase comes in. The neuraminidase, it basically, it nicks, and there's where the N is helpful for remembering it. It nicks, or cuts, sialic acid. And so if it can nick or cut that sialic acid, it can break free. And so I remember the two proteins as hemagglutinin hold sialic acid to enter the cell. That's on entry. And the neuraminidase is going to nick the sialic acid, and that's important for exiting the cell. But we still haven't answered the question, how does all this cause our symptoms? Well, what happens is that as the cells get turned into factories, they start dying, or getting damaged. And all their contents start leaking out. So all these contents from the cell start leaking out. And as they do, they create inflammation. Let me bring up a little bit of canvas. If you have inflammation, let's say that inflammation is happening in your nose. Well, you might say, well, I have a runny nose or a stuffy nose. Or if that inflammation is happening in your throat, you might say, well, I have a sore throat. It might hurt. And if it's happening in your lungs, you might have a cough. So a lot of those respiratory symptoms-- remember we had two categories-- a lot of those respiratory symptoms-- I'm going to shorten as "resp."-- those are going to be explained by inflammation, or at least in part, by inflammation. And remember there are also constitutional symptoms, right? With constitutional symptoms, those are things like having a fever, or having fatigue. And the reason for that is that your immune system is going wild and crazy. When you have influenza, it's going to be attracted to all of those chemicals. We call these cytokines that are being released. And it's going to be attracted to the fact that you've got actual virus particles in the area that's being infected. So that strong immune system is going to create some of your symptoms. It's actually going to rev up your temperature, and you'll start having a fever or chills. And because all your energy is being spent on this attack, you're going to be fatigued. You're going to be fighting off the virus. Since you're going to be fatigued you might have body aches, so a lot of these kinds of symptoms, you get as a result of a strong immune response.