an·a·phy·lax·is 
(ăn′ə-fə-lăksĭs)
(ăn′ə-fə-lăksĭs)n.
A sudden, usually systemic allergic reaction, characterized by vasodilation resulting in decreased blood pressure, smooth muscle contraction resulting in shortness of breath, and hives, caused by hypersensitivity induced by a secondary exposure to a foreign substance, such as a drug or protein, after an initial sensitizing exposure.
[ANA- + (PRO)PHYLAXIS.]
an′a·phy·lactic (-lăktĭk), an′a·phy·lactoid (-toid) adj.
an′a·phy·lacti·cal·ly adv.