**5. Immunogenetics**

Psoriasis is a multifactorial disorder with a strong genetic background. Environmental cofactors play an important role in its manifestation. Various psoriasis susceptibility (PSORS) factors have been identified, of which PSORS1 on chromosome 6p21 has been reproduced in all studies.

Nail psoriasis has more frequently a positive family history as compared to psoriasis of the skin (52.7% vs. 43.8%), is more often associated with psoriatic arthritis (29.7% vs. 11.5%), is more often linked to early onset psoriasis (74.1% vs. 65.5%) and is fewer positive for the HLA allele Cw\*0602 (33% vs. 50.3%) (Armesto et al, 2011, Gudjonsson et al, 2006). It may also be speculated that the IL23R polymorphism that is a common susceptibility factor for psoriasis (Cargill et al, 2007) and is not or only rarely found in Han Chinese may account for the higher rate of familiarity of psoriasis in Caucasians as compared to Asians (Chiam et al, 2011).

Nail psoriasis is associated with a higher frequency of psoriatic arthritis and a more progressive form of the disease (Williamson et al, 2004, Serarslan et al, 2007). The skin and nail lesion usually manifest before the arthritis (Mease , 2002).
