noun. an over-sensitivity to the delivery of an allergen to the body due to prior exposure to the same allergen. The outcome might be centralized or could spread to other areas and spans from welts on the body to seizures, respiratory distress, shock, or even render a person into a coma or death. The outcome is entirely dependent upon the person's degree of sensitivity to the allergen.
ANAPHYLAXIS: "Thomas died as a result of anaphylaxis that occurred when he was stung my multiple bees, developing a severe allergic reaction."