Cystatin-A (Fel d 3) (Fel d 3)

Feline allergen Fel d 3, biochemically identified as Cystatin-A, is a minor allergen produced by the domestic cat (Felis catus). It is an 11 kDa protein belonging to the stefin family of the cystatin superfamily, characterized by its ability to inhibit papain-like cysteine proteases [1, 2, 4]. While its exact physiological role in cats remains to be fully elucidated, it is primarily found in cat dander and skin [1, 5]. In humans, Fel d 3 is a significant cause of IgE-mediated hypersensitivity in a subset of cat-allergic individuals, contributing to clinical symptoms such as allergic rhinitis, conjunctivitis, and asthma [2, 4]. It is frequently employed in component-resolved diagnostics to distinguish specific cat allergies from cross-reactive sensitivities to other mammalian allergens, such as dog cystatin (Can f 8) [2]. Therapeutic management involving Fel d 3 typically includes allergen-specific immunotherapy (SIT), which seeks to desensitize patients by inducing immune tolerance and shifting the immune response toward a non-inflammatory profile [11]. This process involves the induction of regulatory T cells and the production of blocking IgG4 antibodies that prevent the allergen from binding to IgE on mast cells [11, 18]. Although Fel d 1 remains the primary focus of cat allergy research, Fel d 3 represents an important component for comprehensive patient management and diagnostic accuracy [2].

Other names

Cat cystatinStefin AFeline cystatin-ACystatin-A (Felis catus)

Allergen-specific immunotherapy induces immune tolerance by promoting the expansion of regulatory T cells and the production of allergen-specific IgG4 antibodies that block IgE-mediated mast cell activation.