Individual components of our blood, including B cells and T cells, seek out foreign invaders and use their various mechanisms to envelop or destroy them, or both. They stick around for some period of time, some of them for a lifetime, with the “memory” of the invader, so that if it strikes again, the immune system is ready for it without having to ramp up as much as it did the first time it encountered this invasive agent. This is the concept behind vaccines: introduce an attenuated or dead version of the virus so that the body can build up these defenses before the “real” one hits.
