SUMMARY Polymorphic light eruption (PLE) papules were successfully induced on previously affected sites in 11 out of 14 patients with PLE 4–20 h after single exposures to suberythemogenic doses of solar simulated radiation. Histological examination of biopsies performed 1 h, 5 h, 24 h, 72 h and 144 h post‐irradiation revealted onset within 5 h of perivascular cellular infiltration. The infiltrate was dominated by lymphocytes in both early and established lesions, without evident epidermal pathology. Immunohistochemistry demonstrated a predominance of CD4 + cells in lesions up to 72 h post‐induction, but later biopsies were dominated by a CD8 + infiltrate. Significantly increased numbers of dermal macrophages and CD1b + cells were detected 1 h and 5 h post‐irradiation, respectively. These findings are consistent with a delayed type hypersensitivity response underlying the pathogenesis of polymorphic light eruption.

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Summary Eight psoriatic patients undergoing treatment with dithranol were studied. Using cell kinetic and histological data it was shown that retention of dithranol pigment and a return of dermatoglyphics are valid signs of treatment response in psoriasis but that flattening of the plaque with loss of scaling is not. The use of interactive image analysis systems in assessment of histological change is discussed.