Anaphylactic shock is the most severe form of anaphylaxsis, an allergic reaction to a trigger substance known as an allergen. Allergens are commonly things like foods or drugs. Symptoms are related to the release of histamine and other substances into the blood, which act to constrict the airways and relax the blood vessels. The resulting oxygen deprivation and low blood pressure can cause fainting and unconsciousness. Other symptoms can include hives; a flushed appearance; swelling of the face, neck, and throat; vomiting; itching; and anxiety.
In 2373, The Doctor claims that Lieutenant Torres is suffering from delayed anaphylactic shock caused by something she ate when she is found unconscious in Sickbay. In order to fool Captain Janeway and Lieutenant Commander Tuvok, he used cateline to induce Torres into a simulated anaphylactic shock. (VOY: "Darkling")