The allergenicity of the maleic‐modified rosins and their esters has been studied. The unesterified resins arc mainly used in paper size and the esters in printing inks, varnishes and adhesives. The levopimaric‐maleic anhydride Diels‐Alder adduct (maleopimaric acid) is the main component obtained in the maleic‐modified rosins. This compound was synthesized and its structure was determined. Its sensitising potential was investigated in guinea pigs according to different methods. It was shown that maleopimaric acid is a very potent sensitizer, comparable with the strongest allergen isolated from unmodified gum rosin. The allergen may also be present after esterification lion unless the process is carried out to completion. The animals sensitized to maleopimaric acid did not react to unmodified rosin, which shows that maleopimaric acid is structurally different from the allergens i
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