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Current time:0:00Total duration:2:55

Video transcript

there are three groups of influenza viruses which we call virus types and these three types are influenza A B and C so we group influenza viruses like this because of the differences that they have in their genetic material or their genome so the genome in in influenza A virus is very different than the genome in the influenza B virus and there are many more differences between a and B then there are between two influenza A viruses so I'm focusing right now on influenza A and B viruses because these are the types that cause sickness and epidemics every winter in the United States I'm not focusing on influenza type C because this is much less common in humans and actually isn't even included in the annual vaccine so now let's get back to the differences between individual virus types so the influenza type A is a very large group of viruses much larger than the type B and so we need to further divide this group according to subtype and the subtype is is named according to surface proteins that are on the outside of every virus so there are two kinds of surface proteins on every influenza A virus and we call these h4 hemagglutinin and n4 neuraminidase now these surface proteins come in many different flavors there are actually 17 different kinds of hemagglutinin proteins and ten different kinds of neuraminidase proteins so when a virus replicates its genome is going to determine what kinds of H&N proteins will show up on the surface of the virus so you can imagine that there are a lot of different combinations of these viruses so h1 and 1 could be a potential combination maybe h3 and 2 and so on and actually h1n1 and h3n2 are the subtypes that we see in humans today so the combination of proteins that we see in these viruses is very important because this is what the immune system sees when a virus enters the body it sees what's on the outside of the virus so when you have two viruses that are the same or that look the same on the outside and if the immune system knows what to do with one of them it'll know what to do with the other one but if you have a virus come along that looks very different from a virus that the immune system has ever seen before it's going to be very confused and not know what to do with this new virus and when this happens this is what causes major illness across an entire population and we'll learn more about that in a future module