In a joint affected by RA, inflammatory cells of the immune system gather in the lining of the joint (called synovium), forming a fibrous layer of abnormal tissue (called pannus). The pannus releases substances that quicken bone erosion, cartilage destruction and damage to the surrounding ligaments.n a joint affected by RA, inflammatory cells of the immune system gather in the lining of the joint (called synovium), forming a fibrous layer of abnormal tissue (called pannus). The pannus releases substances that quicken bone erosion, cartilage destruction and damage to the surrounding ligaments.