Defective CHS response in ICSBP^-/- mice. Mice were sensitized to FITC on the abdomens and challenged on one ear 6 days later. Ear thickness was measured using an engineers' micrometer 24 hours after challenge on both ears. Ear swelling was calculated by subtracting the thickness of the unchallenged ear from that of the FITC-challenged ear in each individual animal. Control swelling was obtained from mice subjected to challenge but not sensitized. Results are expressed as the mean of at least 5 mice per group ± SE (^*P < .01).