The Late Phase Skin Reaction: Evidence for Activation of the Coagulation System in an IgE-Dependent Reaction in Man

Abstract The late phase reaction (LPR), an IgE-dependent sequel to the wheal and flare response (WFR), was studied in ragweed (RW)-sensitive subjects 15 min and 6 to 8 hr after intradermal RW injection. The role of WFR in the genesis of LPR and LPR dependence on RW dose were determined by skin test titration to extinction of LPR. LPR size correlated with WFR size. LPR were noted with WFR as small as 9.5 ± 1.0 mm (S.E.M.) and RW doses as small as 0.01 protein nitrogen unit (PNU). In a group of WFR-positive but LPR-negative subjects, LPR were not induced despite increasing WFR to 18.2 ± 1.3 mm by simultaneous injection of RW + histamine (RW-H). Light microscopy of 1-µm biopsy sections of direct and Prausnitz-Kustner (P-K) LPR revealed a mixed cellular infiltrate qualitatively no different than that seen 6 hr after isolated WFR. Fibrin deposition in the absence of immunoglobulin or complement was seen by fluorescent microscopy in LPR but not in controls. These data suggest that LPR may follow small RW doses. Furthermore, the failure of RW-H injection to induce LPR argues against a gatekeeper role for WFR in the genesis of LPR. The significance of the cellular infiltrate is not clear although fibrin deposition explains the clinical induration of LPR. This fibrin deposition is evidence for activation of the coagulation system in these IgE-dependent “delayed in time” reactions.