Immune system cells move from the blood into the joints and joint-lining tissue, called synovium. Once they arrive, those immune system cells create inflammation, which wears down cartilage (the cushioning material at the end of bones). As the cartilage wears down, the space between the bones narrows. As it gets worse, the bones could rub against each other.
Inflammation of the joint lining causes swelling and makes fluid build up within the joint. As the lining expands with inflammatory cells, it can produce substances that damage the bone.
All of these things cause the joint to become very painful, swollen, and warm to the touch.