Rheumatoid arthritis. Light micrograph of a section through a rheumatoid nodule, a manifestation of rheumatoid arthritis outside of a joint. There is a large area of necrotic (dead) tissue (bright pink) and an area of palisading histiocytes (immune cells, centre). Rheumatoid arthritis is caused by the immune system attacking the body and its own tissues, causing progressive joint and cartilage destruction.