In a joint, usually two bones are hold together in a capsule which is filled with fluid. The bone ends are covered with cartilage, a layer of cells forming a firm gel-like surface to enable the bone ends to glide smoothly against each other. The cartilage has no blood vessels and thus depends on nutrients from outside. The inner layer of the capsule is covered by the synovial membrane (synovium), a thin layer of cells, which produce the joint fluid. Both, cartilage and joint fluid avoid an abrasion of the bone ends and enable a painless movement.
In an inflamed joint, T lymphocytes have entered the synovium via blood vessels and have released a number of chemical substances which activate a cascade of reactions in the joint. The synovial membrane starts to excessively produce new fluid, which causes a swelling of the joint and can cause an undersupply of nutrients to the cartilage. The cartilage will become thinner by the time and erosions can occur at the area where the synovial membrane touches the bone. All this leads to the painful and sometimes disabling conditions which are typically associated with arthritis.