Origin of threshold behaviour in psoriatic skin.

The mechanisms that lead to the psoriatic morphology are not fully elucidated. Several lines of evidence suggest that the positive feedback between keratinocytes and immunocytes plays a key role in the development of the lesions. On the other hand, little information is available on the negative regulatory controls that maintain a new homoeostasis level in psoriatic skin. We suggest here that the interplay of these two contrary feedbacks is likely to entail a hysteresis effect and that psoriasis is likely to be interpreted as a critical phenomenon characterized by a catastrophic shift of the skin from a normal to a hyperplastic state.