Cat dander is caused by the cats washing themselves. The saliva dries on the hair and then drops off. There is not the same type of thing going on with dogs because they don't groom themselves. Long haired dogs, if they are dirty can carry a lot of pollen etc in their hair, but it's not the same type of allergy.

I have a cat allergy and I miss not having a kitty to snuggle up with. But the last one caused me so much trouble that when the he died, we didn't get another. I have never had any trouble being around dogs.

Agnespuffin said it perfectly. I have the reverse of your daughter, allergic to dogs but not cats. Can't visit my daughter because her roommate has a dog. If I walk into her apartment I start wheezing and having trouble breathing.

Usually people are allergic to cats and not dogs though

To answer your question... because they are allergic to cats, but not dogs. The two are totally different animals, so yes, it is common for people to only be allergic to one of them (usually cats). Just because they are both domesticated animals (and pets) do not mean they are the same animal. Why would you think you'd have to be allergic to both?

A lot more people are allergic to cats than dogs (I think about twice as many).

Here is a link that might be useful: Allergy Foundation

Oh, forgot to add that cat dander is smaller and stickier, so it travels and sticks better and 'sticks' around longer. Also, cats lick themselves more and since saliva is often another allergen, well, you get the picture.

Why would I think you'd probably be allergic to both?

Well, I'm not totally stupid, of course I know it's the saliva, however, although dogs don't exactly "groom" themselves, they certainly do lick themselves on paws, rears & so forth, not to mention slobbering & drooling. Therefore it seems to me there would be dried dog saliva flying around in the air if not more than cat's. I still find that puzzling.

Dog saliva may not be the allegen that a person is allergic to. It isn't the same as that which is produced by a cat. It's the same reason that a person is allergic to the yellow flowers of ragweed, but not the yellow flower of a petunia. Different pollen. Different Allergens. Different people are allergic to different allergens. It's not one of those things that is the same for everyone.

I am a person who has been tested for allergies. I am allergic to cat dander but not to dogs. I had to give away my kitten, thus no more ear infections, sore throats, or eye infections and finally landing in hospital. I am now the proud owner of a dog. I am a very happy camper now. Do not know the difference in cat dander or dog dander but there is a difference, as I have had my dog for awhile now and no problemos. When I had my kitten, I was sick constantly,(ALL THE TIME) thus my Dr. suggested I have some allergy tests done to find out what was going on with me, came back it was my kitty. I was in denial even after test came back until I ended up in hospital with an ear and throat infection so bad it set up a bacterial infection. I had to have my house professional cleaned twice before I could move back in, I was so sick from the kitten. I understand your daughter. I avoid cats and will not even go in a house where there is one.

Thanks Agnes - I see your point though the analogy is a bit stretched (color/flower & dog,cat/animal) however I wonder what the "element", for want of a better description, is in each animal's saliva, I presume especially in cat saliva which creates the difference in allergens if indeed some, though fewer, people are allergic to dog saliva? For that matter, what creates any allergen? I wonder.

I didn't mean to mislead you...I think Dander is usually the main pet allergen, but some people are sensitive to saliva too, while others are sensitive to the urine of some animals like hamsters, gerbils, etc... But, again dander is usually the main problem with cats.

Here's the Wikipedia article on allergies. If you scroll down a bit you'll see a "Basis and Cause" section; that may answer some of your questions.

Here is a link that might be useful: Allergy

It is a protein, and cats and dogs have very different proteins. They have no relationship to each other, except, as someone wrote above, they are both common house pets. A person can be allergic to the protein of one animal and not the other.

Maidenvoyage is correct, most of you need to do your research!

http://www.livescience.com/36578-cat-worse-dogs-allergies-pets.html