All of your cells have the capacity to recognise when they’re infected with a virus. These aren’t just immune cells, they can be skin cells, cells lining your lungs or even your guts.
When a virus is infecting your cells, the virus makes loads of copies itself using our cells molecular machinery(like stealing the cells printer!) and the cell gets filled with the virus’ genetic material; DNA or RNA. Our cells have receptors that can recognise this material building up inside them and they switch on a molecule called Interferon.
Interferon controls loads of anti virus defences inside the cell. Interferon makes the cell break down newly formed DNA and RNA, and stops the cell making protein to stop the virus from making copies of itself and spreading.
Interferon also actives cells of the immune system which come to the site of the infection and kill off infected cells before the infection spreads. These are a mixture of T cells, which are specially equipped to kill virally infected cells and tumours, and B cells, which make antibodies to stop the virus getting into new cells and clump it together to be eaten up by other cells of the immune system.