Question: how does your immune system remember flu? If so does if have different verisons

Craig O'Hare answered on 9 Mar 2017:

When we make an immune response to a virus we build an army of cells called T cells. T cells are really good at killing cells infected with viruses. They live in special places called lymph nodes.

When other cells of the immune system get infected with flu, they show it to the T cells. The T cell best able to recognise the virus then starts to divide rapidly to build a force strong enough to fight against the virus. This causes swelling in the lymph nodes, which can you sometimes can feel as pain in your neck. The infected cells send out a signal which says they’re infected called a chemokine. The T cells follow this chemokine trail to the infected cells and remove them along with the virus.

Most of these T cells die off when the virus infection has cleared . However some of them will hang around in our bodies for a few years. These are called memory T cells, and if we get sick again with the same virus, they can expand rapidly to build another army of T cells again but only faster than when we first got infected.

The reason flu is still a problem is because it grows very fast and mutates really quickly. So even though our immune systems catch up to the virus, the virus is always changing so it outruns the immune system. It’s so good at doing this, all the flu scientists have to get together every year to try guess which way it might mutate so we can make a vaccine to limit how sick everyone gets. You might have heard of different flues being called H1N1 or H5N8 these names are used to distinguish different mutants of flu