The desensitization of rabbits sensitized to complex proteins, such as human serum, was considered in a previous article (1). It was shown that, after an intravenous injection of a moderate amount of specific protein in a sensitized animal, no skin sensitivity reactions are obtained and the serum precipitins apparently disappear from the circulation; also that this desensitization phase is of comparatively short duration, that within twenty-four hours after the intravenous injection the animal begins to show a tendency to return to the sensitized state. Early experiments dealing with specific desensitization of rabbits that are sensitized (or immunized) to different organisms led to several observations which were not identical to those made in connection with desensitization to proteins. It was noted, for example, that an intravenous desensitizing injection of a suspension of typhoid bacilli in rabbits sensitized to these organisms, caused a reduction in the skin sensitivity response, but produced no apparent effect on the serum agglutinins.

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