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SUMMARY Mast cells in the upper dermis and mucosal subepithelial layer were counted in thirty consecutive light microscopy oil-immersion fields of the following: thirty-four skin and oral specimens from lesions in patients with active Behcet's disease, eight specimens of apparently uninvolved skin of the same patients, and 102 lesions of a variety of other skin diseases. The results revealed a significant increase in the number of mast cells in Behcet's lesions. Furthermore, the histamine content of five reactive lesions showed a two-fold increase compared with that in apparently uninvolved skin of patients with active Behcet's disease. It is suggested that mechanisms similar to cutaneous basophil hypersensitivity may be involved in the production of Behcet's lesions.

SUMMARY The mechanism of skin hyperreactivity (pathergy) in Behcet's disease is unknown. It has been suggested that the response is due to an Arthus-like reaction. In 18 patients with active Behcet's disease, pathergy was induced and the skin biopsied after 24 h. The histological reaction was that of mild to intense round cell infiltration, perivascular in type. Direct immunofluorescence failed in all 18 patients to demonstrate the local presence of immunoglobulins and complement at the site of lesion. A marked increase in the number of mast cells was observed at the site of reaction and scattered throughout the dermis, the role of which is to be further investigated. The study failed to demonstrate a humoral mechanism in the production of the cutaneous hyperreactivity.