When in 1981 Rob Aalberse from the University of Amsterdam noticed the enormous cross-reactivity of some patients´ sera against virtually any plant and even insects, notably, insect venoms, it took ten years to arrive at a possible structural explanation of this phenomenon. 1991, Japanese researchers determined the structure of the epitope common to horseradish peroxidase and Drosophila neurons as being an asparagine-linked oligosaccharide (N-glycan) containing a xylose and a core-linked α1,3-linked fucose residue. These structural features are not present in humans and animals. Core α1,3-fucose was then found to be relevant for the binding of patients´ IgE to honeybee venom allergens, which contain N-glycans with structural similarities to plant N-glycans. Ever since then, core α1,3-fucose emerged as the structural element most relevant as a CCD in plants and insect allergens. Much later, both xylose and core α1,3-fucose were revealed as heart pieces of two independent glycan epitopes for rabbit IgG. The occurrence of human anti-xylose IgE, however, has not been verified so far. Still, because of the two possible epitopes and the different carrier structures, the plural CCDs is in frequent use even though core α1,3-fucose appears to be the single culprit.