Feline serum albumin (Fel d 2) (Fel d 2)

Feline allergen Fel d 2, also known as feline serum albumin (FSA), is a 69 kDa protein and a significant allergen in domestic cats (Felis catus) (UniProt P49064). While Fel d 1 is the primary cat allergen, Fel d 2 is recognized by approximately 15-25% of cat-allergic individuals and is the primary cause of "pork-cat syndrome," where patients experience allergic reactions to pork due to cross-reactivity between feline and porcine serum albumins (PubMed 11453871). Biologically, Fel d 2 serves as a major carrier protein in feline blood, responsible for transporting various ligands such as fatty acids, hormones, and drugs, while also maintaining the osmotic pressure of the plasma (PubMed 15985814). In the context of allergic disease, it acts as an antigen that triggers IgE-mediated hypersensitivity, leading to symptoms of rhinitis, conjunctivitis, and asthma (WHO/IUIS Allergen Nomenclature). Therapeutic intervention typically involves allergen-specific immunotherapy (AIT), which uses standardized cat allergen extracts containing Fel d 2 to desensitize the immune system (NIH: StatPearls). Monitoring Fel d 2-specific IgE is a critical diagnostic step in identifying patients at risk for multi-mammalian meat allergies and tailoring immunotherapy protocols (PubMed 11453871). The high degree of sequence conservation among mammalian albumins explains the frequent cross-reactivity observed in patients sensitized to Fel d 2 (PubMed 15985814). Unlike Fel d 1, which is primarily produced in salivary and sebaceous glands, Fel d 2 is a blood-derived protein that can be found in dander, urine, and saliva (PubMed 10474034).

Other names

Feline serum albuminFSAAlbuminFel d II

Allergen-specific immunotherapy (AIT) utilizes the Fel d 2 allergen to induce immune tolerance by shifting the immune response from a Th2-mediated allergic profile to a Th1-mediated or regulatory T-cell (Treg) profile, increasing the production of IL-10 and TGF-beta, and inducing the formation of allergen-specific IgG4 antibodies that block IgE-mediated mast cell activation (PubMed 21453361).