Experts are not sure what the exact cause of psoriasis is. We know the immune system plays a part, and we also know there is excessive growth and reproduction of skin cells.
Fault of the epidermis
One hypothesis is that psoriasis is mainly a fault of the epidermis (the upper or outer layer of skin) and its keratinocytes. The keratinocyte is the major constituent of the epidermis, making up 95% of the cells found there. Keratinocytes are cells found in the epidermis - those at the outer surface of the epidermis are dead and form a tough protective layer, while the cells below divide and replenish the supply.
An immune-mediated disorder
Another hypothesis is that the excessive reproduction of skin cells is secondary, and that the main factor is related to an immune system disorder.
Our immune system is designed to protect us from foreign bodies and pathogens (things that cause disease), such as bacteria, viruses, and toxic substances. An autoimmune reaction occurs when the immune system mistakes a normal or good substance for a pathogen, and attacks it. Sometimes our immune system may attack good tissues and cells in our bodies; this is called an autoimmune disease.
Our T-cells (T lymphocytes) normally help protect the body against infection - they are a type of white blood cell and form part of our immune system. T cells travel throughout the human body to detect and fight off foreign substances, such as bacteria or viruses. If a person has psoriasis, however, the T cells attack healthy skin cells by mistake. Experts believe these T-cells become active, migrate to the dermis (inner/deeper layer of skin) and trigger the release of cytokines, in particular tumor necrosis factor-alpha (TNFα). TNFα is what causes the excessive production of skin cells, as well as inflammation.
Experts do not know what triggers the activation of T-cells - genetic and environmental factors are most likely, they say. The result is a cycle of skin cell production becoming faster and faster. It normally takes 28 days for skin cells to be created and then to die - this cycle is reduced to 2 to 6 days in patients with psoriasis, causing dead skin cells to accumulate on the surface of the skin, in thick scaly patches.