I have read some articles where people were saying that whether you
would build up immunity or beome hypersensitive to bee venom would
depend in great extent to the frequency and dosage of venom you are
exposed to. According to those articles, beekeepers, being exposed very
frequently to relatively large dosages, would build up immunity. Their
wifes, on the other hand, being exposed unfrequntly and to low dosages
(as venom crystals in gloves etc) would tend to become hypersensitive.
  I don't know how accurate those articles were, but, if their authors
were wright, being stung twice a year might be too close to the boundary