LEARN MORE ABOUT
ALLERGENS
Fel d1: The Primary Allergen in Cats
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Most cat-related allergic reactions are triggered by a small, stable glycoprotein known as Fel d1. Each allergen identified within a species is assigned a sequential number, and the first allergen discovered in domestic cats was designated Feline domesticus allergen 1 (Fel d1).
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Fel d1 is secreted by the lacrimal, salivary, sebaceous, and perianal glands. During grooming, the allergen present in saliva is spread onto the fur and can easily become airborne. Meanwhile, allergen secreted through the skin glands adheres to the coat and can be readily transferred to clothing, carpets, and upholstered furniture. Among household environments, litter boxes typically contain the highest concentrations of Fel d1, as this allergen is especially abundant in the anal glands.
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The length of a cat’s coat does not affect the total amount of allergen produced. However, the fine undercoat hair of some breeds, such as Siberians, may cause facial itching unrelated to Fel d1 levels. Although shedding occurs throughout the year, it is most pronounced during the early spring.
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Factors Influencing Fel d1 Production
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Scientific research has demonstrated that neutering male cats significantly reduces Fel d1 levels, as the allergen is hormonally regulated. Unneutered males produce the highest concentrations, whereas neutered males and females generally have similar allergen outputs.
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In a 2006 study involving 420 unneutered adult Siberian cats, approximately half exhibited lower Fel d1 levels than typical domestic cats. About 15% of these Siberians had exceptionally low allergen concentrations, with both males and females represented in this low-allergen group.
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Interestingly, Silver and Smoke-colored Siberians tend to have Fel d1 levels two to four times higher than the breed average. Initial findings from 2005 confirmed this pattern, while more recent data (2023) from a breeding line of low-allergen Silver Siberians suggest that the Silver gene, a relatively recent introduction into the breed, may be genetically linked to higher Fel d1 expression. With the availability of DNA testing for the Silver gene introduced in 2023, the genetic relationship between coat color and allergen levels may soon be better understood.
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Studies examining more than 200 Siberian matings revealed that even within low-allergen pairings, only one or two kittens per litter showed extremely low Fel d1 levels, while others exhibited moderate or typical concentrations. Conversely, when both parents had normal allergen levels, all offspring demonstrated high Fel d1 production.
Secondary Allergens
Cat allergies are complex, and no single allergen is responsible for all reactions. To date, eight distinct feline allergens have been identified in saliva, fur, urine, and feces. While sensitivities vary between individuals, most allergic people react to multiple allergens.
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Approximately 20% of individuals with severe allergies to horses, dogs, rabbits, or even certain foods (such as pork, beef, or egg whites) may tolerate low–Fel d1 cats without significant reactions. The remaining 80% experience symptoms that range from mild to severe, depending on personal sensitivity.
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Among secondary allergens, Fel d2 (feline albumin) and Fel d4 (feline lipocalin) are the most common. Serum albumin is responsible for many cross-reactions between cats and other animal species, while lipocalin is often associated with asthmatic or airway responses.
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For example, around 25% of horse-allergic individuals also react to Fel d4, a condition known as horse/cat cross-reactivity. In general, cat-induced asthma is primarily caused by either Fel d1 or Fel d4.
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Studies in mice have shown that intact males produce 500–1000 times more lipocalin than females, and these levels drop sharply after neutering. Although comparable studies in cats are limited, it is reasonable to infer that neutered males or females may provoke less severe reactions in individuals sensitive to lipocalin.
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For those with severe cat-induced asthma, it is highly recommended to spend time with a low–Fel d1 cat before adoption. Testing reactions using fur samples can be helpful but may not fully predict responses in individuals who are also allergic to lipocalin or albumin.
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Prevalence of Feline Allergens
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The chart below (not included here) summarizes the most prevalent feline allergens and the approximate percentage of cat-allergic individuals who react to each.
