Cat Allergy in humans is an allergic reaction to one of the following feline allergens (1):


AllergenClass of Allergen
Fel d 1
Uteroglobin   (chain 1)
Fel d 2
Albumin
Fel d 3
Cystatin 
Fel d 4
Lipocalin  
Fel d 5w
Feline Immunoglobulin A    (F IgA)
Fel d 6w
Feline Immunoglobulin M   (F IgM)
Fel d 7
von Ebner gland protein

The most common of these are the glycoprotein Fel d 1 (2), created largely in saliva and sebaceous glands of the cat. It is spread onto the cat's fur during grooming and once dry, it easily becomes airborne. It is therefore not the cat's fur that causes an allergic reaction and hence longhaired or shorthaired and even hairless cats can all cause reactions in allergic individuals. The protein is of an unknown function to the animal but causes an IgG or IgE reaction in sensitive humans (either as an allergic or asthmatic response).
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Fel d 1 is incredibly lightweight and sticky, so it remains suspended in the air for a long while and then settles and clings to everything: carpets, walls, furniture, clothing, draperies. Cat allergens are "stickier" than most of the other types of allergen. Once a cat is in a home, a "reservoir" of allergen quickly develops in and on the furnishings, and as there is movement in the house, the allergen gets stirred up and airborne again. In homes with cats, there is a relatively stable amount of allergen around all the time, constantly replenished by the reservoir in the environment, as well as by the cat.
Different cat races produce different amount of Fel d 1 and i
ndividual cats produce different amounts of Fel d 1, but there are a few generalizations that can be made: Intact males produce Fel d 1 in levels higher than castrated males, and the assumption is that Fel d 1 is hormonally regulated by testosterone (3). Neutered males produce Fel d 1 in levels similar to females (both intact and spayed females produce Fel d 1 in similar levels). Even though females and neutered males produce Fel d 1 in lower levels, they still produce enough to cause allergic symptoms in sensitive individuals. The level of Fel d 1 does however seem to increase as the cat matures and hormones increase and thus spaying/neutering will reduce the allergen levels produced by the cat. The increase of Fel d1 with age may explain why some people adopt a kitten only to find that they are allergic when the kitten grows up! However when living with a cat the repeated exposure to cat allergens may also reduce an individual's reaction to the cat.
If there are allergies to other animals as well as cats, chances are that there will still be an allergic reaction to a cat that has a low level of Fel d 1 as it is likely one of the other proteins causes the allergic reaction. The Siberian Research Inc. (4) believes that if there is an allergic reaction to cats and not to any other animals then it is most likely an allergic reaction to the Fel d 1. They believe that Fel d 1 accounts for around 60% of allergic reactions to cats.
References
1
Allergen Nomenclature Database The Allergen Nomenclature Sub-committee operates under the auspices of the International Union of Immunological Societies (I.U.I.S.) and the World Health Organization (W.H.O.). This is an excellent database on allergens from mammals, plants, foods, and other sources.
2 Clinical Immunology and Allergy Unit, Department of Medicine, Karolinska Institutet and University Hospital, Stockholm, Sweden. (http://content.karger.com/ProdukteDB/produkte.asp?Aktion=ShowFulltext&ArtikelNr=250435&Ausgabe=253625&ProduktNr=224161#SA2)
3 Sex difference in Fel d 1 allergen production. http://www.jacionline.org/article/S0091-6749(96)70238-5/fulltext.
4 The Siberian Research Inc. http://siberianresearch.com/
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