Psoriasis is an autoimmune disease of the skin.

Psoriasis is an autoimmune disease of the skin. This disease occurs when one of the main features of the immune system – recognition between self and non-self — is compromised.

We shed skin periodically as an immunity measure for the body. When cells of the first layer are shed, the second layer is already beneath it. Since this takes place on a microscopic level, we do not notice it. However, in psoriasis, our brain gets confused and identifies our own skin as a potential threat to our body. Therefore, T-cells attack our skin at various points on our body. This causes a major inflammation and in order the heal the damage, the skin undergoes rapid growth which leads to over accumulation of skin layers. Since the upper layers haven’t been shed yet and there are already multiple layers formed, the skin  appears scaly and reddish at various places, similar to a rash. Having such rashes all over the body due to major confusion in our immune system is known as psoriasis.
People afflicted with Psoriasis exhibit the following symptoms:

Rashes
Scaly red inflamed skin
Flaky scalp and skin
Pitted fingernails

Know more: https://www.psoriasis.org/