Histamine weal formation and absorption in man.

1 The weal and flare response to intradermal histamine was measured over a range of doses in forearm skin of man. 2 The use of calipers to measure weal thickness was validated by measurement of observer and method errors. Repeated measurements at intervals of 5 min or more compressed the weals by 6% per reading. 3 Flare response reached a maximum at 5 min compared with 20 min for weal thickness and the sensitivity of the flare response was greater than the weal response at lower doses of histamine. 4 Sensitivity, lambda was comparable for the measurement of flare, weal diameter and thickness or volume. The advantage of weal thickness was in the measurement of weal formation and resorption. 5 The time course of histamine weal formation and disappearance was measured and found to have a T1/2 of 5.4 min and 87 min, respectively. 6 The T1/2 of disappearance of comparable 09% w/v NaCl solution and serum weals were 18 and 28 min respectively. 7 It is suggested that persistence of histamine weals is due to a vasoactive agent other than histamine.