Eight 2,4-dinitrophenyl compounds with a high degree of configurational uniformity were examined in regard to their ability to elicit delayed allergic skin reactions in guinea pigs and men who had been sensitized by previous exposure to 2,4-dinitrofluorobenzene. Four of the eight compounds produced reactions (elicitors) and four others did not (non-elicitors). It was found that the elicitors combined with protein in vitro and in vivo. The non-elicitors, however, failed to combine in either situation. It is concluded that a necessary condition for the elicitation of delayed allergic skin reactions by haptens is the combination of the latter with skin protein through the formation of bonds of the covalent type. No choice is yet possible amongst the several possible explanations which are advanced to account for the obligatory character of protein combination.

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