Polcalcins such as Bet v 4 and Phl p 7 are pollen allergens that are con-structed from EF-hand motifs, which are very common and well character-ized helix–loop–helix motifs with calcium-binding functions, as elementary building blocks. Being members of an exceptionally well-characterized protein superfamily, these allergens highlight the fundamental challenge in explaining what features distinguish allergens from nonallergenic proteins.