Due to popular demand, I will now differentiate between Rheumatoid Arthritis (RA) and Osteoarthritis (OA). Don't worry, I ll keep the technobabble to a minimum and use colorful pictures as appropriate.
OA is a mechanical form of joint destruction on weight bearing joints whereas RA is an inflammatory form of joint destruction.
In the blue picture to the left, an inflamed knee joint is represented by an enlarged synovium (sac that nourishes cartilage). The pathphysiology of RA is that autoantibodies (antibodies against components of your own bodies) form complexes with cartilage, other antibodies and likely still other anatomical features. These complexes activate complement, a pathway of inflammation in the body. The ongoing inflammation results in recruitment of inflammatory cells that like to eat bone (desorption) and the production of structures called Pannus that invade the cartilage space and change cell signaling in the joint.
Nobody really knows why these autoantibodies form but everyone has their own theory. The one that I like most is that infectious particles (viruses or bacteria) establish an infection at some point in the patient's life and the immune system reacts appropriately making antibodies to the infection. However, things go wrong when B cells (cells that produce antibodies) start pumping out less faithful copies of these antibodies (epitope spreading) that coincidently bind to self. These B cells are "helped" by errant T cells possibly through dysregulation of T cell ontogeny (T cell maturation).
Other great explanations for this autoimmune disorder are environmental (smoking, systemic inflammation), genetic (T cell receptors that present self antigens a bit too readily), and others.
Other types of arthritis also result in joint damage but have different mechanisms and different causes.
Woo, that was full of technical language. Hope it was nonetheless clear
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