Observations on the Formation of Wheals. II. Comparison of Wheals Induced by Allergens and by Histamin

Abstract

The lesion of allergy (used in the sense of atopy) is essentially a localized edema, whether or not smooth muscle spasm occurs also. The edema of the bronchi in asthma, the swelling of the nasal mucosa in hay fever, the wheal of urticaria and the positive skin reaction in an allergic individual all show the same histological picture. With the hope of throwing some light on tissue participation in allergy, a study of wheal formation was undertaken.

In a previous publication, 1 it was shown that when intradermal tests were done with an offending allergen at different sites in an allergic individual, the resulting wheal varied greatly in size. This occurred despite the fact that the dose of the injected allergen was constant (0.02 ccm.). Wheals laid down on the back and abdomen were uniformly much larger than those on the extremities, and those on the upper outer thigh were intermediate in size.

The wheal of a positive skin test results presumably from a reaction between the injected allergen, and antibody in the skin tissues, with the consequent formation of some substance having a resemblance to histamin. This, in turn, acts directly upon the walls of the surrounding capillaries. These dilate and become permeable to their contained plasma which comes out into the tissues and forms the wheal.

In order to analyze which of these factors may be responsible for the discrepancy in the size of wheals in different sites on the skin in response to a constant amount of allergen, the following experiment was performed:

Normal individuals were injected intradermally with 0.02 can. of a 1 to 2500 dilution of histamin phosphate in the interscapular region, the flexor surface of the left forearm and in the upper outer aspect of the right thigh.