This article is from the Canine Allergies FAQ, by Cheryl Minnier with numerous contributions by others.
This type of reaction, again usually severe itching, is not to the
flea itself but rather to proteins in its saliva. Dr. Ackerman writes
that dogs most prone to this problem, interestingly enough, are not
dogs who are constantly flea ridden, but those who are exposed only
occasionally! A single bite can cause a reaction for five to seven
days, so you don't need a lot of fleas to have a miserable dog.
To test for flea allergies, a skin test is performed which must be
read in fifteen minutes and again in forty eight hours. Unfortunately
injections to desensitize are not very effective because it is hard to
collect enough flea saliva to make a serum!
For dogs with this problem a strict flea control regime must be
maintained. We would caution you, however, against using strong
chemical preparations on your dog. Often times the flea control
program produces more harmful effects than the fleas, including
seizures and skin problems, so please use caution.