Failure of isotretinoin to control dermatitis herpetiformis and subcorneal pustular dermatosis

both stem and leaf, consist of a minute capillary tube, silicified at its upper end and calcified at its lower end, enclosed at the tip in the shape of the bulb. The latter breaks off after contact with the skin, exposing a fine needle-like point which penetrates the skin, pressure leading to expression of the fluid contents into the dermis. Although initial studies proposed formic and acetic acid as mediators^ subsequent investigation confirmed that nettle hairs contain pharmacologically active concentrations of histamine and acetyl choline.^ They also showed that intradermal co-injection of histamine and acetyl choline in the same concentrations reproduced the nettle sting response. Collier and Chesher* identified a third amine, 5hydroxytryptamine, in nettle hairs. Several investigators have confirmed these pharmacological findings in other members of the nettle family world-wide.'"* Saxena et al.^ also showed that the nettle hairs of Urtica parviflora contained a histamine-liberating substance. Although histamine, acetyl choline and 5hydroxytryptamine collectively may well account for the vascular changes and immediate discomfort due to nettle stings, the striking and persistent paraesthesiae remain unexplained and suggest the involvement of hitherto unidentified neurotoxic factors. In any event, it is highly unlikely that formic acid plays a role in the pathophysiology of this common, but intriguing contact urticaria.