This study was designed to evaluate the specificity of the skin reactions induced by intradermal injections of various antigens. The intradermal skin tests were carried out with the crude extracts of hookworm, house dust and ragweed in 118 and 484 individuals in the rural districts of Tokyo and Shizuoka Prefecture. These antigens were extracted with D. P. solution and diluted with physiological saline. The dilution of the injected were 1:10,000 in hookworm, 1:1,000 in house dust, and 1:1,000 in ragweed. As the control, physiological saline and a 1:10,000,000 solution of histamine hydrochloride diluted with physiological saline were used. One twentieth ml. of each solution was injected in the flexor surface of forearms. All measured reactions were of immediate type, 15-20 minutes after injections. Each wheal and erythema was measured in two directions (paralleled to arm length and transverse of it), and the average of two readings was described as the diameter of the wheal and erythema. 1) The distribution of the diameter of wheal produced by the physiological saline in 222 individuals showed a normal distribution and their mean and standard deviation were 6.4±1.38 mm. The distribution of erythema showed likely logarithmic one. 2) Each of these three antigens showed two similar peaks in the distribution curves of the wheal and erythema. The first peak in the smaller diameter side resembled that caused by the physiological saline concerning both wheal and erythema. The second peak in the larger diameter side corresponded to the group which was sensitized by specific antigens. Concerning hookworm infection and the growth of ragweed, there were more chances of antigen contact in the district of Tokyo than that of Shizuoka Prefecture. Comparison with the result of the each district revealed that the second peaks of the distribution curves both in wheal and erythema were significantly higher in the former district. 3) The three antigens tested in the same population showed the similar distribution curves of the diameters of wheal and erythema with the two peaks, and with the minimal points between the two peaks situated in the same part. From these results it seemed reasonable that the boderline diameters between negative and positive reactions be set at the same point; 9 mm in wheal and 20 mm in erythema. When these standard were apllied to the reactions of hookworm patients, more than 90% of them were included in the range of positive reaction. 4) When the measurement was done only on the reaction caused by antigen, the false positive reaction might occur in hypersensitive persons to some unspecific irritants. So that, the diameters of antigen reactions both in wheal and erythema were compared with the differences of diameters between the antigen and saline reactions, and the correlation coefficients between those measurement were+0.8〜+0.9 in wheal and+0.96〜+0.99 in erythema concerning three antigens respectively. Negative reactions to antigens were mostly same to the saline reactions both in wheal and erythema (over 70% of them). 5) Wheal reaction was not always proportional to the erythema, though a high correlation (r=+0.7) was observed between them. Then it may be rational that the positive response must be judged by the simultaneous measurement of wheal and erythema. 6) A slight correlation was observed between the diameters of wheal and/or erythema of three antigens and those of histamine (r=+0.35).
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