2.3.1.1　Etiology

The etiology of psoriasis is not yet fully elucidated.Currently，psoriasis is recognized as polygenic disease mediated by genetic and environmental factors.

1.Genetic factors

All studies on demographic survey，pedigree，genetics（e.g.，twin studies），and HLA strongly suggest a genetic predisposition to psoriasis.About 20％of psoriatic patients report a positive family history，of whom psoriasis occurs at an earlier age than those without family history.The chance for an offspring to develop psoriasis is about 16％if one parent is affected by the disease and the likelihood goes up to 50％when both parents are affected.Histocompatibility antigen studies have shown that the class Ⅰ antigens A1，A13，A28，B13，B17，B37，and Cw6（in which Cw6 shows the strongest association with this disease），and the class Ⅱ antigen DR7 are more likely to be expressed in psoriatic patients compared to normal individuals.

2.Environmental factors(https://www.daowen.com)

Epidemiological studies for the prevalence of psoriasis in monozygotic twins suggest a 70％chance of one twin developing psoriasis if the other has developed the condition，but only a 20％chance for dizygotic twins.These findings indicate that the hereditary factors are insufficient to trigger psoriasis independently，while the environmental factors play an important role in its occurrence.The common environmental factors triggering or aggravating psoriasis include infection，emotional stress，trauma，surgical incision，pregnancy，cigarette smoking，and certain kinds of drugs.

3.Immune factors

Abundant lymphocytes and monocytes infiltrate lesions of psoriasis vulgaris，especially T cells.It is an important pathological feature of psoriasis，indicating T cells involved in the occurrence and development of the disease.However，the role of T cells in the pathogenesis remains unclear.