Causes
Psoriasis and psoriatic arthritis are inflammatory conditions of the seronegative type. That means that blood autoantibodies (antibodies, types of protein made by the immune system, which attack a person's own body cells and tissues) found in other autoimmune diseases are usually absent. Nevertheless, psoriasis belongs to the same basic class of diseases as rheumatoid arthritis, which is usually a seropositive disease.
Researchers don't know exactly what causes psoriatic arthritis, but they believe it is autoimmune in nature. In these conditions the body's immune system mistakenly attacks its own tissue. Most experts believe this is probably triggered by an infection or changes in the environment, though the exact mechanism as to how and why this occurs is still not known.
Genetics and a family history of psoriasis and psoriatic arthritis play a role in the condition. 40% of people with psoriasis have a first-degree relative with the same disease. Despite the strong genetic role, psoriatic arthritis sometimes appears in children with no family history of the disease. About 15% of people with psoriasis develop psoriatic arthritis as well.