In the case of RA, a trigger is simply something that drives the development of the disease in people who are genetically predisposed to it. It can be an infection, a behavior, even a hormonal change. Whatever it is, the trigger sets off production of abnormal proteins called citrullinated peptides, explains Robert Hylland, M.D., a rheumatologist in Muskegon, MI. Our immune system identifies these proteins as foreign invaders and mounts an attack of antibodies, resulting in inflammation. Some inflammation is a good thing: After all, it’s our body’s natural weapon against viruses, wounds, cancer cells, or other foreign invaders. But with RA, Dr. Hylland explains, the body starts attacking its own cells in our joints, causing swelling, stiffness, and a host of other symptoms. Luckily, scientists have ID’d several triggers for RA so you can avoid flare ups and live as pain-free as possible.