Allergen Profile, Cat, IgE with component reflex *

Importantly, cat (Felis domesticus) dander allergic individuals are sensitized to a heterogeneous range of cat allergens.^6,16 The major cat allergen is the secretoglobin Fel d 1,¹⁶ which accounts for majority all IgE reactivity to cat dander.^17,18 All cat allergens other than Fel d 1 are considered minor allergens because less than fifty percent of cat dander sensitized individuals test positive for sIgE to these proteins. The minor cat allergens for which we have the most clinical data are the serum albumin, Fel d 2 and the lipocalin, Fel d 4.

Fel d 1

Fel d 1 is produced by the sebaceous glands cat and is found in their saliva.^16,19 This a thermostable protein is therefore present in large concentrations on skin surface and fur of cats as a result of grooming.²⁰ Cats are the only mammals for which a secretoglobulin is the primary sensitizer for allergic disease. No comparable protein has been characterized in other species. Since Fel d 1 does not have significant IgE cross-reactivity with other mammalian proteins, Fel d 1 positivity is considered to denote cat specific sensitization.²⁰ Serum-specific Immunoglobulin E (sIgE) to Fel d 1 is present in approximately 95% of patients that test positive for sIgE to cat dander.^18,21-26 Early studies showed 60-90% of overall allergenic activity to cat dander is directed against Fel d 1.¹⁶ Depending on the population tested, 40-70% percent of Fel d 1 sIgE positive patients are not sensitized to any other cat component routinely measured (Fel d 2 and Fel d 4).^23,24

A large European study of more than 4000 children eight years old and younger (the ABAMSE/MeDALL study) reported that sIgE reactivity to the three cat allergen molecules tested (Fel d 1, Fel d and Fel d 4) increased with age.²⁵ Fel d 1 was the dominant sensitizing cat allergen during the entire childhood in this population.²⁵ Consistent with other studies,^5,21,27 the ABAMSE/MeDALL study found that testing for sIgE to Fel d 1 was comparable to testing for sIgE to cat allergen extract in diagnosing cat allergy.²⁵ This study also found that sIgE to Fel d 1 in young children was more predictive of cat allergy at 16 years of age than measuring sIgE to cat allergen extract.²⁵ In the ABAMSE/MeDALL population, less than ten percent of patients were positive for Fel d 2 and/or Fel d 4 and not Fel d 1.²⁵ Testing for sIgE to Fel d 1 directly, without testing for sIgE to cat dander extract would have caused a small number of cases with non-typical sensitization profiles (about 5-10%) to be missed.^25,28

Sensitization to cats and dogs in childhood has been shown to predict the persistence of asthma throughout the teen ages ¹¹ and cat dander has been shown to be a dominant sensitizer in young adults with asthma.¹⁰ In a population-based study of 963 19-year-old Swedes, levels of sIgE to Fel d 1 were found to be strongly associated with the prevalence, severity, and persistence of asthma.²⁹ This finding was consistent with a other studies that have found that increased Fel d 1 sIgE levels can serve as a prognostic marker of future cat allergy and allergic asthma.^{2,3,21,24,25,27,30,46}

Fel d 2

The protein Fel d 2 is cat albumin.³¹ In the context of respiratory allergy, serum albumins are considered to be an uncommon cause of allergic sensitization.^31-33 Sensitization to Fel d 2 has been associated with moderate/severe rhinitis and a diagnosis of asthma in patients sensitized to other cat components.^3,26,34 Wisniewski et al. described an association between high levels of sIgE to Fel d 2 and atopic dermatitis in cat-allergic children.³⁵

Serum albumins are major allergen in meats, which underlies their importance as food allergens.³³ Most albumin sensitized patients tolerate well-cooked meat or milk as serum albumins are thermolabile. Cross-reactivity between cat and pork albumin is thought to be responsible for allergic reactions after ingestion of pork in cat-allergic individuals in a condition referred to as pork-cat-syndrome.^33,36-37 It is thought that initial environmental exposure to cat albumin causes sensitization that results in subsequent rapid onset of allergic reactions on eating pork.^36,37