First, some definitions:

Allergen:  A protein that is not harmful, but yet causes a reaction in your body as if it were.  Usually a naturally occuring protein from a plant, animal, or fungus that has a useful purpose for that creature.  Sometimes, can be a man made protein (like a medication).

Sensitivity:  When a protein in your enviroment causes an allergic reaction

Tolerance:  When a protein in your environment that used to cause an allergic reaction, no longer gives you as much trouble.

And now some fun facts about pet allergies:

Pet allergens are everywhere.  The cat allergens have been the most studied and have been found in schools, in drycleaned items even if those items did not have cat allergen on them before being given to the drycleaner. And in brand new mattresses.  Buy your mattress with plastic on it and avoid floor samples or you will bring home allergens as well.

A lot of people are co-sensitized to dogs and cats, or allergic to both.

You can decrease the amount of allergen shedding by washing your dog but in the best titled study I have found so far:  Washing the dog reduces dog allergen levels, but the dog needs to be washed twice a week (Hodson et al Journal of Allergy and Clinical Immunology.  103(4):581-5)  What more do you need to know?  Really?

I got a lot of great information on these issues from

Morris DO, Human allergy to environmental pet danders:  a public health perspective Veterinary Dermatology 2010 March 30  Epub. ahead of print