What is difference between plasma cells and memory cells, and how do they prevent you becoming ill?

The main difference between plasma cells and memory cells is WHEN they are active in fighting infection.

Plasma cells are immediately active on defense during the first exposure to invaders like viruses or bacteria producing antibodies (proteins that identify or neutralize the foreign invaders). Memory cells last longer and are imprinted by the offending antigens of the invaders, and so are ready to mount a more rapid defense later, during a second exposure.