Psoriasis is a systemic disease that affects multiple organs, usually the skin and joints, as well as the auditory, cardiovascular, and endocrine systems.

It affects around two percent of the U.S. population, and has a strong immunogenic element. Most people with psoriasis are diagnosed before the age of 40, while very few people are diagnosed later in life.

The form that occurs before the age of 40 is strongly associated with the human leukocyte antigen (HLA) class I HLA-Cw6 The pathogenesis of psoriasis has yet to be identified.

One hypothesis is infectious Staphylococcus and Streptococcus species have been known to induce one form of psoriasis known as guttate psoriasis.

The immunopathology of psoriasis begins with antigen presentation in an environment high in interferon γ (IFNγ), interleukin (IL)6, and IL1β to T-cells in draining lymph nodes, where IL12, IL23, and tumor necrosis factor (TNF) α are in high concentration.

From this, TH22, TH1, and TH17 white blood cells proliferate. The pro-inflammatory T-cells return to the skin's dermal layer and release pro-inflammatory molecules.