I don't know that you can become not allergic to dogs, but you can try reducing your exposure/symptoms. If you know when you are going to be around a dog, you could try taking an allergy medication such as benadryl or tavist-d. I've found that for myself, it's best to avoid dogs altogether. If I am in a house where there is an inside dog, after a few minutes my eyes start stinging and watering. And if I happen to come in contact with dog slobber, I break out into hives.
There are instances that allergies is not really an allergy. It is caused by the mind set that we input to our mind. This problem is can be overcome by being used to dog smell and to the presence of dog.