triple response, a triad of phenomena that occur in sequence after an intradermal injection of histamine. First, a red spot develops, spreading outward for a few millimeters, reaching its maximal size within 1 minute and then turning bluish. Next, a brighter red flush of color spreads slowly in an irregular flare around the original red spot. Finally, a wheal filled with fluid forms over the original spot. Also called triple response of Lewis.
