Allergic reaction to bee stings occurs when a person becomes sensitized to the venom from a previous sting. This reaction is different from the reaction to the poison in the bite of a black widow spider, which injects a potent toxin into the blood. Ordinarily, bee venom is not toxic and will only cause local pain and swelling. The allergic reaction comes when the immune system is oversensitized to the venom and produces antibodies to it. Histamines and other substances are released into the bloodstream, causing blood vessels to dilate and tissues to swell. Severe reactions can lead to anaphylactic shock, a life-threatening series of symptoms including swelling of the throat and difficulty breathing. Persons who develop an allergy to bee stings should carry prescription bee sting kits to counteract the reaction to bee venom.