Causes
Psoriasis and psoriatic arthritis are inflammatory conditions of the seronegative type. That means a certain marker - rheumatoid factor - usually isn't found in blood tests. Nevertheless, psoriasis belongs to the same basic class of diseases as rheumatoid arthritis, which is 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. If a child has parents with psoriasis, the child is three times more likely to develop the condition. Despite the strong genetic role, psoriatic arthritis sometimes appears in children with no family history of the disease. Statistics show that up to 30% of people who develop psoriasis will later develop psoriatic arthritis as well.