From MedlinePlus Genetics
Ankylosing spondylitis is a form of painful, ongoing joint inflammation (chronic inflammatory arthritis) that primarily affects the spine.  Early symptoms of ankylosing spondylitis typically begin between the ages of 15 and 30. Most commonly, affected individuals first experience chronic back pain and stiffness. This pain worsens with rest or inactivity, and tends to be relieved with physical activity or exercise.

Pain in ankylosing spondylitis results from inflammation of the joints between the pelvic bones (the ilia) and the base of the spine (the sacrum). These joints are called sacroiliac joints, and inflammation of these joints is known as sacroiliitis. The inflammation gradually spreads to the joints between the vertebrae, eventually involving the whole spine, causing a condition called spondylitis. Over time, back movement gradually becomes limited as the bones of the spine (vertebrae) fuse together. This progressive bony fusion is called ankylosis. These fused bones are prone to fracture.

Ankylosing spondylitis can involve other joints as well, including the shoulders, hips, and, less often, the knees. As the disease progresses, it can affect the joints between the spine and ribs, restricting movement of the chest and making it difficult to breathe deeply. 

Ankylosing spondylitis affects the eyes in more than 30 percent of cases, leading to episodes of eye inflammation called acute iritis. Acute iritis typically affects one eye at a time and causes eye pain and increased sensitivity to light (photophobia). Rarely, ankylosing spondylitis can also cause serious complications involving the heart, lungs, and nervous system. Six to 10 percent of people with ankylosing spondylitis have additional inflammatory disorders such as psoriasis, which affects the skin, or ulcerative colitis or Crohn's disease, which both affect the digestive tract.  https://medlineplus.gov/genetics/condition/ankylosing-spondylitis