White blood cells, the agents of the immune system, travel to the synovium and cause inflammation (synovitis), characterized by warmth, redness, swelling, and pain
—typical symptoms of rheumatoid arthritis
. During the inflammation process, the normally thin synovium becomes thick and makes the joint swollen and puffy to the touch.
As rheumatoid arthritis progresses, the inflamed synovium invades and destroys the cartilage and bone within the joint.