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Summary Pollens of closely related species often share common antigens and allergens although this is not always the case. Four species of Oleaceae pollens commonly found in the Mediterranean area were investigated For cross‐reactivity: olive (Olea europaea), ash (Fraxinus exselsior), privet (Ligustrum vulagare) and Phillyreu augustifolia, a common bush. Twenty individual sera of patients allergic to Oleaceae were investigated for specific IgE antibodies against the four species of Oleaceae pollens. The results indicated a high degree of correlation between RAST titres of Oleaceae pollens but three gave a particular emphasis on one species only. Cross‐reactivity among these four pollen species was sought by means of RAST inhibition, iso‐electric focusing and tandem cross‐immunoelectrophoresis. All tests revealed a high degree of cross reactivity although there is no total identity among these four pollen species.

Pollens of related species usually share common antigens and are cross-reacting but this is not always the case. The cross-allergenicity of two species of Urticaceae pollens {Parietaria and Urtica, nettle) was sought by means of RAST-inhibition and isoelectric focusing. A preliminary experiment in forty-two patients investigated by means of skin tests and RAST showed that most patients were either allergic to Parietaria or nettle and that both reactivities were uncommonly associated. The results of RAST-inhibition confirmed the total absence of cross-reactivity between these two related species. IEF patterns indicated that most proteins are in the acidic range of pH and did not reveal large discrepancies. Parietaria and Urtica pollens, although belonging to the same family and having close microscopic patterns, have very different allergenic activities. This study demonstrates that cross-allergenicity cannot be predicted.

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