The alum-precipitated pyridine extracts of allergenic materials recently introduced for desensitization contain much more alum in the precipitate than is commonly used in most vaccines. A study in rabbits of the relationship of alum concentration to antigen retention and antibody responses showed that an increase in alum concentration to 30 mg. per milliliter resulted in more prolonged retention of antigen at the site of injection than is observed with mineral oil-Arlacel emulsions. When used to elicit secondary immune responses, these very persistent precipitates showed little adjwvant effect and gave antibody responses only slightly better than saline solutions of antigen. Such alum precipitates, however, may provide an even greater safety factor than oil emulsions to prevent systemic reaction to toxic antigens.
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