IL1B and Arthritis: Moreover, using this animal model, it was observed that deficiency of the IL-1ra (a naturally occurring inhibitor of IL-1) gene induced autoimmunity and arthritis, with erosive destruction of the ankle bone among other features, such as infiltration of inflammatory cells, proliferation of the lining cells in the synovial membrane, and neutrophil infiltration, emphasizing the importance of IL-1/IL-1ra balance in maintaining joints physiology and immune system homeostasis (99).