The body's immune system usually defends against illness. But in lupus, the immune system mistakenly attacks healthy cells and tissues, damaging joints, skin, blood vessels and organs. There are many types of lupus, but most often the term refers to systemic lupus erythematosus (SLE). SLE can be hard to diagnose. People with SLE don't have the exact same problems, and the symptoms may be like those of other conditions, including juvenile rheumatoid arthritis.