The exact cause of AS is unknown, but it’s considered to be a systemic, or whole body, “autoimmune” disease—a disease in which the immune system attacks other parts of the body. In the case of AS, the immune system attacks the spine and other areas of the body, causing the joints, tendons, and ligaments to become inflamed, stiff, and painful. Usually, the inflammation begins where the spine meets the pelvis, in a place called the sacroiliac joint.