The levels and function of Tregs, memory T cells, neutrophils, mast cells, Langerhans cells, cytokine and chemokine profiles, vitamin D levels in the blood and/or skin will be studied in PLE patients compared to control groups. Volunteers of four groups will be enrolled in this study: i) patients with PLE undergoing preventive medical UV photohardening in spring; ii) PLE patients not undergoing preventive UV photohardening; iii) healthy control subjects; and iv) patients with other diseases (including psoriasis, atopic dermatitis, and other conditions) undergoing therapeutic phototherapy.