Experts still debate the exact causes of psoriasis

The disease clearly results from the excessive growth and reproduction of deep-layer skin cells called keratinocytes.

According to a popular hypothesis, psoriasis is an autoimmune disease. The immune system mistakenly targets some of its own cells for attack. Here we see white blood cells (T-cells) migrate from the bloodstream to the epidermis of the skin, where they release inflammation-promoting proteins called cytokines.

Among the most damaging of these is tumour necrosis factor-alpha (TNF-a). TNF-a triggers inflammation and promotes excessive growth and accumulation of skin cells, which die as they pile up on the surface, forming silvery plaques which is by far the most common type, affecting about 85% of people with the disease.

Plaque psoriasis manifests itself as reddish to whitish scaly patches, often appearing on the skin of the hands, knees, elbows, scalp and lower back. The raised, inflamed, silvery-white patches are called plaques.

Guttate psoriasis is a less common variant of the disease, in which many small, round, red or pink spots form over large areas of the body. Inverse psoriasis is a form that features smooth reddish patches that often appear in folds of skin, such as under the arms or between the thighs and groin.