Positive atopy patch test reactions to Pityrosporum orbiculare in atopic dermatitis patients

background Pityrosporum orbiculare, although a part of our normal cutaneous microflora, can cause skin infections and induce specific immunoglobulin ( Ig) E antibodies in atopic dermatitis patients. P. orbiculare is therefore considered to be one of the trigger factors for atopic dermatitis. Objective To investigate if P, orbiculare can induce an eczematous reaction in atopic dermatitis patients, seborrhoeic dermatitis patients and healthy controls. Methods Fifteen atopic dermatitis patients, eight seborrhoeic dermatitis pa tients and eight healthy controls were patch tested with extract of P. orbi culare on non-lesional, tape-stripped skin of the back. NaCl was used as a negative control. The patch tests were evaluated after 24, 48 and 72 h. Ski n biopsies were taken from P. orbiculare patch test sites at 24 h and 72 h, from NaCl patch test sites at 72 h, from non-lesional skin rind, in the at opic dermatitis patients, also from lesional skin. The skin biopsies were i nvestigated with immunohistochemical techniques. P. orbiculare-specific IgE in serum was analysed with RAST. Results Specific IgE to P. orbiculare was found in serum from 13/15 atopic dermatitis patients and in eight of them a positive patch test reaction to P. orbiculare was observed, with a maximal reaction at 48 h. Significantly higher serum levels of P. orbiculare-specific IgE were detected in patch te st-positive compared with patch test-negative atopic dermatitis patients (P < 0.01). The seborrhoeic dermatitis patients and healthy controls were RAS T and patch test-negative for P. orbiculare. In the patch test-positive ato pic dermatitis patients an infiltration of CD4(+) T cells and eosinophils w as observed at the P. orbiculare patch test sites together with an upregula tion of ICAM-1 and HLA-DR expression. Conclusions P. orbiculare can induce an eczematous reaction in sensitized a topic dermatitis patients and may be an important trigger factor in these p atients. The P. orbiculare patch test can be of diagnostic value in this su bgroup of atopic dermatitis patients.