When a B cell encounters its triggering antigen, it initiates the development of plasma cells, large cells that produce millions of antibodies identical to those on the parent B cell. These antibodies pour into the bloodstream and bind to the triggering antigens, so they can be recognized and destroyed by:

1. Natural killer (NK) cells, white blood cells that contain granules with lethal enzymes. The natural killer cell binds to the antigen, injects it with the enzymes, and kills it, unless there are signals to turn the NK cell off.
2. Macrophages, large white blood cells that engulf and digest antigens.
3. The activation of complement, a complex series of blood proteins whose action "complements" the work of antibodies. Each component of the complement system is activated in turn in a precise sequence of events.

When a cytotoxic T cell recognizes its triggering antigen, it binds to the antigen, releasing lethal substances that perforate the cell membrane and kill the cell.