What action should I take if I think someone is having a severe allergic reaction ?

The most important step to take is to call 911 if you
think someone is having a severe allergic reaction.  Many people with very serious
allergies carry an injection kit with them for such emergencies, but still, a person
with such a reaction should be examined and treated by a doctor, quickly.  Benedryl can
also be given and can help until experienced medical personnel can address the
problem.

I do want to emphasize that you would want to be
very sure that the person was experiencing an allergic reaction.  If you are with
someone you know, who has a serious allergy, you will probably be aware of that and know
the signs.  If you are with someone whom you do not know, trying to help that person in
any way could be a risky thing to do.

I have provided a
link for you that describes the symptoms of the most severe kind of allergic reaction,
anaphylaxis, so you can see why it is always imperative that you seek medical
attention.