1. In immunology, the process that stimulates resting (nonfunctional) white blood cells to assume their role in the immune response. The process involves recognition of an antigen or a response to cytokines.
2. In neuropsychology, arousal.
3. In physiology, the triggering of a cell response, as when a neurotransmitter causes ionic channels to open in an excitable cell membrane, setting off an action potential.

See: antigen processing; cytokine; immune response