Johnny starts to cry. His mother tells him the shot keeps him from getting sick. But he doesn't understand. Most of us don't. Helper T cells spot the antigens, latch on and send out signals calling for the production of B cells and Killer T cells.
B cells produce antibodies that destroy free-floating viruses.
Killer T's seek out and destroy viruses that are inside other cells. Once the infection is destroyed, memory cells, acting like sentries, remain on guard in case the infection returns. Little Johnny had chickenpox last year. He has memory cells that are specially trained to fight chickenpox. He won't get it again.