The following allergens have been characterised:
- Car b 1, a 17 kD Bet v 1 homologue (4-10).
- Car b 2, a profilin (6).
- Car b CBP, a calcium-binding protein (6,11).
Cross Reactivity
An extensive cross-reactivity among the different individual species of the genus could be expected (12). Indeed, Birch (Betula verrucosa), Alder (Alnus glutinosa), Hazel (Corylus avellana), and Horn beam (Carpinus betulus) all belong to the order Fagales, and a high degree of homology of their 4 major allergens has been indicated (13). This is mostly due to pollen from these trees containing allergens that share IgE epitopes with Bet v 1 and Bet v 2 (6-8,12).
The deduced amino acid sequences in Car b 1 from Horn beam pollen show pronounced homology with Bet v 1, the major allergen from Betula verrucosa (White birch) pollen (5,10).
Homologues to Bet v 1, the major Birch pollen allergen, are also present in members of the Fagaceae, Rosaceae and Apiaceae families (2-14,15).
As Horn beam tree pollen contains profilin and calcium-binding proteins, cross-reactivity may occur with other pollens containing these panallergens (6,11).
The Horn beam tree contains an N-glycan carbohydrate structure. It is theorised that carbohydrate structures are another potential source of immunological cross-reaction among allergens such as Kentucky blue grass (Poa pratensis), Rye (Secale cerale), Rye grass (Lolium perenne), Short ragweed (Ambrosia elatior), Giant ragweed (Ambrosia trifida), Birch (Betula alba), Horse chestnut (Aesculus hippocastanum), Olive (Olea europaea) and snake-skin pine (Pinus leucodermis) pollen (16).
Patients with Type I allergy to Fagales pollens frequently show adverse reactions to fruits and vegetables, in particular to Apple and to Hazelnut (17).