When the immune system detects a virus or infection, it goes into overdrive and attacks it. An immune response is what this is referred to as immune reactions.
A person who had chickenpox or been immunized against chickenpox, for example, is immune to getting it again.
White blood cells :
White blood cells scour the blood and tissues of your body for foreign invaders (microbes) like bacteria, viruses, pathogens, and fungi.
They initiate an immune attack when they find them.