Psoriatic arthritis can happen if you have psoriasis, a condition that causes your skin to get red patches and silvery scales. Sometimes people get psoriasis first and then have psoriatic arthritis later, but others get the arthritis symptoms first. Like lupus, psoriatic arthritis happens when your immune system attacks your healthy cells and tissues. This causes your joints to get inflamed and your skin to make too many cells. It's not clear what causes the immune system to attack your body when you have psoriatic arthritis, but just like lupus, it's likely that genes and triggers play a role.