Fig. 1: Schematic representation of a biphasic anaphylactic reaction. The second-phase reaction has been described as occurring between 1 and 8 hours after the initial reaction, but new evidence suggests that this second phase may occur up to 38 hours (mean 10 hours) after the initial reaction. About one-third of the second-phase reactions are more severe, one-third are as severe and one-third are less severe.