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.  The fine downy fur from the undercoat of Siberians can cause itching of the face 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.

In most cases, Silver/Smoke Siberians have Fel d1 allergen levels much higher than the average Siberian.  Our first studies in 2005 showed the levels of allergen in Silver Siberians (Silver/Smoke/Bi-Metalic) 2-4x higher than most Siberians.  Recently in 2023, we tested adults from a cattery with a line of low allergen Silver Siberians.  The Silver color is a more recent introduction into the Siberian breed, and there some degree of linkage equilibrium between Fel d1 and Silver appears to be present.   This might become more clear in the coming years, as DNA tests for Silver were introduced in 2023. 

We 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, eight different cat allergens have been isolated, and are found in cat saliva, fur, urine, and feces. These allergens may cause a reaction in one person but not another, although most people are allergic to more than one cat allergen.

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

Anyone with severe cat induced asthma should spend time with a low-Fel d1 allergen cat before purchasing a kitten. Testing with fur samples is a good starting point, but may be insufficient 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 d5 Feline Immunoglobulin A (F IgA)
Fel d6 Feline Immunoglobulin M (F IgM)
Fel d7 Lipocalin: von Ebner gland protein (asthma / excema common)

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

© 2005  KittenTesting

\© Meredith Lundberg 2005 - Lundberg Siberians