Mike Rowbottom asked, "Is it possible that different sub-species of
bees have slightly different venom make-up, and that it is possible
to be immune to some bee stings, but not to others?"

The short answer is yes.  But I suspect the reaction your wife had
was a normal reaction.  You wrote she had built up an immunity to
bee stings.  Such an immunity (tolerance would be a better word)
doesn't last forever, it requires regular doses of venom to be
maintained.  Assuming you wife went through a desensitizing
regimen, if there was a good length of time between her last
treatment (or sting) and the sting that caused the reaction, it just
may have been due to timing.  Even beekeepers who receive
regular stings throughout the season can experience more
pronounced reactions to the first sting of spring than they will to
just another sting in the summer.

Aaron Morris - thinking a sting a day keeps the allergist away!