IL-3 is required for hapten specific priming. (A) IL-3–deficient (□) and wild-type littermate controls (▪) (BALB/c) were tested for contact hypersensitivity to oxazolone. One hundred nanograms of murine IL-3 (in 1% mouse serum) (□) or vehicle only was administered intraperitoneally and subcutaneously (abdomen) 4 hours before, at the time of sensitization, and 6 hours afterwards. +/+ versus −/−, P = .03. −/− plus murine IL-3 versus −/−, P = .008. Murine IL-3 administered to unsensitized mice had no effect on secondary challenge. (B through D) Histopathology (tissues were formalin-fixed, paraffin-embedded, and stained with hematoxylin and eosin) of secondary oxazolone challenge sites in ears from mice (BALB/c background) killed 24 hours after challenge. (B) Ear reaction of a sensitized wild-type mouse. (C) Ear reaction of a sensitized IL-3–deficient mouse. (D) Ear reaction of a sensitized IL-3–deficient mouse administered IL-3 protein at the time of priming.