Concomitant patch test reactions to nickel and palladium have frequently been reported in patients undergoing investigation because of suspected allergic contact dermatitis. Theoretically, these reactions can be explained by multiple, concomitant, simultaneous sensitization as well as cross-sensitization. We studied whether concomitant reactions to nickel and palladium could represent cross-sensitization in females hypersensitive to combinations of nickel, palladium and cobalt. Females were patch tested with serial dilutions of nickel sulfate, cobalt chloride and palladium chloride on the upper back. 1 month later, when the patch test reactions were gone, the patients were randomized into 2 groups that were challenged orally with either nickel or placebo. 1 day later, the areas of previous positive patch test reactions were read in a blind way looking for flare-up reactions. Nickel provocation but not placebo yielded flare-up reactions on sites previously tested with nickel (P = 0.012) and palladium (P = 0.006), but were also observed on sites previously tested with cobalt, even though this was not statistically significant. Flare-up reactions of previous patch test reactions to nickel and palladium after oral challenge with nickel speak in favour of a cross-reactivity mechanism.

Ambrosia artemisiifolia L. a classical cause of ragweed dermatitis in North America, is also found in Europe. 17 patients with contact allergy to sesquiterpene lactone (SL) mix were recalled for patch testing with extracts of North American and Swedish ragweed, the latter harvested in summer and autumn. 8/17 patients were test-positive to the American extract, 14/17 to the Swedish summer extract and 15/17 to the autumn extract. All patients except 1 were found to have a chronic hand eczema of the pompholyx type. 5/16 hand eczema patients had a flare-up of their pompholyx at patch testing. We concluded that patients with SL mix allergy are allergic to American as well as to Swedish ragweed. Pompholyx seems to be a frequent clinical picture in patients with SL mix allergy.