When you tell your allergist you think you're allergic to your cat, she'll test you with Cat Dander E1. Sure, she believes you; but that vial of dander will confirm it's really the cat causing your symptoms. Your allergy could be caused by something else.
Cat Dander E1 is one common name of the compound used to test people for cat allergies. It's a special form of cat dander administered during routine allergy tests. Such tests can confirm whether you have a special kind of Immunoglobulin E -- a blood compound involved with immunity and allergic reactions -- that targets cat dander. It's usually one specific protein in the cat dander, Fel D1, that causes cat-dander-specific Immunoglobulin E antibodies to go into overdrive and cause allergies. Essentially, this cat allergy is an over-reactive immune system response. Cat Dander E1 can help confirm a cat allergy, although the sneezing and runny nose you experienced after petting a cat probably already tipped you off.
It's a good thing your cat doesn't have problems with cat dander, because he's covered in the stuff. That's not because cat hair or shed cat skin cells -- which are what dander is -- create the allergen. They don't. The Fel D1 protein responsible for the lion's share of cat allergies is in your cat's saliva. Cats are fastidious groomers, which is why Fel D1 ends up all over their bodies. When your cat sheds hair or skin, the allergen enters the cat's and your environment. Cat Dander E1 contains that allergen, which is why it serves in allergy tests. Technically, you could remove the allergen from the compound or cat hair, but they're practically inseparable in the real world.
All cats produce Fel D1, so there's no such thing as a truly hypoallergenic cat. That's why one type of cat dander -- namely, Cat Dander E1 -- is sufficient for allergy tests. Some breeds of cats with less hair appear to be less likely to cause allergic reactions in sensitive people. Devon Rex, Cornish Rex and Sphynx breeds are some of the most popular. Designer cats engineered in 2006 by a company called Allerca may not produce Fel D1, although the 100-percent-hypoallergenic claim was unproven at the time of publication. Even the allergens of so-called hypoallergenic cats can stick around for months, so you should keep up on assisted grooming and vacuuming to mitigate allergic reactions.
Scant scientific data exists regardingo why, but some common ideas do exist about attributes that make a cat less allergenic, which is to say it produces less Fel D1. Siberian long-haired cats may produce less of the protein in question. Light-colored cats of any breed seem to produce less of it than dark-colored cats. Kittens, females and neutered males appear to produce less of it than their intact adult male counterparts. If any of these ideas proves true for you, your reaction to Cat Dander E1 may not be an accurate gauge of how you'll fare with a specific cat. As long as your allergies aren't deadly serious, you may be able to control your reactions by controlling your cat's environment.