Flu Pathophysiology


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 an
influenza A virus is very different than the
genome in the influenza B virus, and there are many more
differences between A and B than 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 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 H for hemagglutinin
and N for neuroaminidase. Now, these surface proteins
come in many different flavors. There are actually 17 different
kinds of hemagglutinin proteins and 10 different kinds of
neuroaminidase proteins. So when a virus
replicates, its genome is going to determine what
kinds of H and 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 H1N1 could be a potential
combination, maybe H3N2, 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.

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