New aspects of the biology of the mast cell and its associated products may be important to the pathogenesis of arthritis. Mast cells may be activated in human arthritides as a result of a rapidly expanding list of peptides, including cytokines and neurotransmitters. The classic immunoglobulin E-coupled mast cell activation pathways may be used in the autoimmune diseases by way of autoantibody production involving the immunoglobulin E molecule itself. This can be shown to induce mediator release in vitro. The mast cell-derived products may play a role in both the early phases of synovitis as well as the chronic destructive phase.