H1N1, H3N2, H1N7; we hear their names daily, but have you ever wondered what they mean? Why do we refer to the different strains of influenza by a ‘H’ and ‘N’ followed by a series of seemingly random numbers?
In order to replicate (reproduction is the ultimate ‘aim’ of all organisms!), viruses must first enter a host’s cell. In order to achieve this the virus must first interact with molecules on the host’s surface. These molecules must fit together like a lock and a key. If either one is the wrong shape, the virus won’t be able to enter the cell. As I described in ‘MERS CoV – will the Time-Bomb Explode?‘, this is especially important when looking at viruses which evolve to infect new species.
The letter ‘H’ stands for haemagglutinin, the ‘key’ on the virus. It binds to the ‘lock’ on the host cell, a molecule called sialic acid, and then enters. Interestingly, sialic acid is only found on animal lung cells, which is why we must inhale the virus to become infected!
The letter ‘N’ stands for neuraminidase. This is the ‘key’ which lets the virus back out of the cell. The virus needs to exit again so that it can go on to infect the next cell!
The haemagglutinin and neuraminidase proteins can vary, and each strain of virus will have one type of each. The number of each has been allocated purely based on the order in which it was discovered, so H1N1 has the first haemagglutinin and first neuraminidase proteins to be discovered.
That’s all very well, but why name a virus after these two proteins? The reason is that these proteins are the proteins that can most easily be ‘seen’ by the host’s immune system. Antibodies are made against these proteins, and this is what allows for the creation of immunity. That’s why being infected with H2N7 won’t protect you against H3N2 infection, and why scientists have to make informed guesses as to which strains should be vaccinated against in any one year.