Abstract
Psoriasis is a common non-communicable chronic immune-mediated skin disease, affecting approximately 125 million people in the world. Its pathogenesis results from a combination of genetic and environmental factors. The pathogenesis of psoriasis seems to be driven by the interaction between innate immune cells, adaptive immune cells and keratinocytes, in a process mediated by cytokines (including interleukins (IL)-6, IL-17 and IL-22, interferon and tumor necrosis factor) and other signaling molecules. This leads to an inflammatory process with increased proliferation of epidermal cells, neo-angiogenesis and infiltration of dendritic cells in the skin. Dysfunctional de novo glucocorticoid synthesis in psoriatic keratinocytes and the skin microbiome have also been suggested as mediators in the pathogenesis of this disease. To understand psoriasis, it is essential to comprehend the processes underlying the skin immunity and neuroendocrinology. This review paper focuses on the skin as a neuroendocrine organ and summarizes what is known about the skin immune system, the brain–skin connection and the role played by the serotonergic system in skin. Subsequently, the alterations of neuroimmune processes and of the serotonergic system in psoriatic skin are discussed, as well as, briefly, the genetic basis of psoriasis.
Highlights
The skin is an essential organ, the largest of the human body
Psoriasis is clinically classified in five subtypes: psoriasis vulgaris or plaque psoriasis, the most common form, affecting 90% of the patients [13]; psoriasis guttate, the second most common form, characterized by small, droplet-shaped lesions that appear on the trunk, arms and legs; pustular psoriasis, a rare clinical subtype of psoriasis where the skin shows diffuse redness and subcorneal pustules [75]; erythrodermic psoriasis, a rare and severe variant of the disease, characterized by a generalized inflammatory erythema in at least 75% of the skin [76]; and inverse psoriasis or flexural psoriasis, a form of plaque psoriasis that affects inverse/intertriginous/flexural body areas, such as ears, axillae, groin and clefts [77]
This review focuses on the skin as a complex organ, with immune, endocrine and neurological functions similar to the brain
Summary
Psoriasis is an immune-mediated disease that causes raised, red, scaly patches to appear on the skin. It is one of the most common chronic non-communicable diseases [71], affecting approximately. The most common form of psoriasis is chronic plaque psoriasis, which is characterized by stable and localized erythematous scaly plaques that are well demarcated from normal skin. These range from very few to a high number covering a large area of the skin. Other less known comorbidities of psoriasis have been described, such as cardiovascular disease, psoriatic arthritis, sleep apnea, osteoporosis and celiac disease [82]
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