Blood will be taken by venous puncture (mainly of a cubital vein) from the individual study participants at four defined time points during the year: (i) spring (March to April) (before medical photohardening in PLE patients); (ii) spring/early summer (April to June) (immediately after medical photohardening of PLE patients); (iii) late summer (August to September); and (iv) late fall (November to December). In addition, optional skin biopsies will be taken to study the parameters listed above. The statistical power analysis (alpha 0.05; power 0.8; assumed difference in Treg level/function of 30% among groups; based on the data by Myara et al., 2005) revealed that 23 patients (21+2 expected drop-outs) need to be enrolled per patient group. All patients of the non-PLE groups will be sex- and age (plus/minus 5 years)-matched to the PLE subjects.