Fel D1:  

The majority of cat allergies are caused by a small stable glycoprotein called Fel d1.   Each allergen in a species is given a number when it is identified.  The first house cat allergen identified was labeled Feline domesticus allergen 1, or (Fel d 1).

Fel-d1 is produced by lacrimal, salivary, sebaceous, and perianal glands. During grooming, Fel-d1 from saliva becomes airborne. Allergen from the skin glands stays on the fur and is easily transferred to clothing, carpet, and upholstery.   Litter boxes often contain the highest amount of Fel d1, as the highest concentrations are found in the anal glands.

Fur length does not increase or decrease the amount of allergen produced.  Long fur does allows the cat to groom more allergen off the coat which can lower reactions. The fine downy fur from the undercoat can cause itching which is not related to Fel d1 allergen levels.  Shedding is most common during early spring, but occurs all year.

Multiple studies have shown neutering sharply reduces the levels of Fel-d 1 in males. The allergen is hormonally controlled, with non-neutered males producing the highest amounts. Genetically low males and females produce similar amounts of Fel d1.

In 2006, allergen levels were measured in 420 adult un-neutered Siberian cats. Half of the Siberians in the study had found to have lower levels of feline allergen than other domestic breeds.  Roughly 15% of the cats had exceptionally low levels of Fel d1, with both males and females found in the lowest group.

Silver/Smoke Siberians consistently showed higher levels of Fel d1 than other colors.  The reason is unknown, but silver may be carried on the same chromosome as  Fel d1.  A few low Fel d1 silver cats were documented, but not enough to be statically valid.

We have studied over 200 matings of Siberian cats in an effort to understand the allergen genetics. In very low-allergen matings, one or two kittens in each litter had exceptionally low levels of Fel d1, but even in these litters, some kittens had moderate to normal allergen levels. When cats with normal levels were mated, all kittens had normal high levels of allergen.


There are no simple answers to cat allergies.  To date, thirteen cat serums and eight cat danders that can cause allergic reactions have been isolated. Any of these allergens can cause a reaction in one person but not another, and most people are allergic to more than one cat allergen.

About 20% of individuals with severe allergies to horses, dogs, rabbits, (or food allergies to pork meat, beef, or egg white) have no reaction around a low Fel d1 cat.  Reactions in the other 80% will vary by individual, and can range from very mild to severe.

Most secondary cat allergies are caused by Fel d2 (feline albumin) or Fel d4 (feline lipocalin).  Serum albumin accounts for about the largest number of cross reactions between cats and other animals.

About 25% of people who are allergic to horses also react to Fel d4.  These individuals are considered  horse/cat cross-reactive. Asthma and other airway reactions to animals are usually caused by lipocalin. When asthma is caused by exposure to cats, it is typically caused by Fel d4 (feline lipocalin) or Fel d1 (androgen binding glycoprotein).

Several studies have shown that intact male mice produce 500 to 1000 more lipocalin than female mice. Levels of lipocalin in male and female mice are sharply reduced by neutering. These studies have not been performed on cats, but it is reasonable to assume that individuals with severe horse or rabbit allergies will have less reaction to a female or neutered male kitten.

At this time, there are are no commercial tests that can measure Fel d4 or most other cat allergens. Anyone with severe cat induced asthma should spend time with a known low-Fel d1 allergen cat before purchasing a kitten. Testing with fur samples is not sufficient for people with lipocalin or albumin allergies.

This chart illustrates the commonest feline allergens, and shows the rough percentage of cat-allergic people who react to each individual allergen.

Allergen Class Percent of Reaction
Fel d1 Secretoglobin 88-95% (always causes asthma)
Fel d2 Serum Albumin 20-35% (often severe)
Fel d3 Cystatin
Fel d4 Lipocalin (asthma / excema common)
Fel d5w Feline Immunoglobulin A (F IgA)
Fel d6w Feline Immunoglobulin M (F IgM)
Fel d7 von Ebner gland protein

Reference reading for secondary allergens. 

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.).  They have an excellent database on allergens from mammals, plants, foods, and other sources.

Cat Dander 
Thermo Fisher Scientific Inc has published a nice technical article on feline allergens and some cross reactions with other animals.

The Major Cat Allergen, Fel d 1, in Diagnosis and Therapy
Some of the most recent work in feline allergens has been performed by the Clinical Immunology and Allergy Unit, Department of Medicine, Karolinska Institutet and University Hospital, Stockholm, Sweden.

Fel d 4,  A Cat Lipocalin Allergen
Lipocalin may be a major cause of initial sensitization to cat.  This technical study on feline lipocalins is available on

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\© Meredith Lundberg 2005 - Lundberg Siberians