**1. Introduction**

8 Psoriasis

Zhang K, Liu R, Yin G, et al. Differential cytokine secretion of cultured bone marrow

2010; 20 (1): 1-5.

stromal cells from patients with psoriasis and healthy volunteers. Eur J Dermatol

Psoriasis is a chronic, inflammatory skin disease affecting 2 to 3% of the white population (Gudjonsson & Elder, 2007). It is a multifactorial disease since its development depends on a complex interplay of genetic and environmental factors. As no pathogen has been consistently identified within psoriatic plaques (indeed skin infections are rare in lesions because of antimicrobial peptides) (Nomura et al., 2003), an autoimmune basis for the chronic inflammation is the dogma for this complex disorder. Psoriasis is characterized by macroscopic (clinical) and corresponding microscopic (histological) skin alterations and leads to considerable impairment of the quality of life of the affected patients. Special forms of psoriasis (e.g. arthropathic form) can be accompanied by severe extra-cutaneous changes.
