If you know you have a severe allergy that might cause anaphylactic shock, you carry a self-injecting device that doses you with epinephrine (adrenaline), called an Epi-Pen. The problem exists when you don't know you have an allergy. Bee stings are the most common example of an unknown allergy. Most people with severe peanut allergies, know they're allergic given how common peanut products are.

Both my children have peanut allergies, but not so severe that exposure to peanuts triggers the reaction. However, our allergist says that some people do have such a severe allergy that mere exposure can trigger a reaction.

As a previous poster mentions, the physical allergy has been documented in peer-reviewed journals.