>Isn't it also the case that rather than building up an immunity that
>one can develope severe reactions to stings where no reactions were
>evident previously?
 
Yes, although I'm told that near-fatal reactions are rare compared with
the former.  Still, it probably wouldn't be a bad idea to carry a "bee
sting kit" (Anakit) or an epinephrine auto-injector (Epipen) when
working with bees.  Both require a prescription from your physician.
Some may think that this is being overly cautious.  I look at as being
an inexpensive form of insurance.