1. an excessive responsiveness of the immune system to certain foreign substances, including various drugs. Hypersensitivity reactions may be immediate, involving an acute allergic reaction leading to anaphylaxis, or more delayed, involving dangerous and sometimes fatal reductions in the number of certain white blood cells (see agranulocytosis) in response to treatment with some antipsychotic drugs (clozapine is a classic example). Drug hypersensitivity can also result in serum-sickness-type reactions or in an immune vasculitis, such as Stevens–Johnson syndrome, as seen after administration of some anticonvulsant drugs.

2. an extreme responsiveness to sensory stimuli (e.g., sound, light, touch).

3. a tendency toward emotional overreaction to criticism, rejection, or other social judgment. See also sensitivity.