Psoriatic arthritis, like psoriasis, is an autoimmune condition. That means both are caused by your immune system. When your immune system turns itself on automatically and starts attacking healthy cells and tissues instead of battling bacteria and viruses, the result can be the inflammation common in both conditions. In psoriasis, this inflammation happens in the skin, and in psoriatic arthritis this inflammation leads to swollen and painful joints and tendons.
Unfortunately, researchers still don’t know exactly what triggers autoimmunity, or even how it works.
What is clear, however, is that the process is similar to how the body responds to real invaders: it tries to get rid of them by using a type of immune cell, called T-cells, to begin an inflammatory attack. When you’re sick or injured, this is a good thing, as it triggers inflammation and causes blood vessels to expand. For people with psoriasis, this unwelcome attack results in red, itchy and scaly patches on the skin, and for those with psoriatic arthritis, pain and stiffness in the joints.