Rheumatoid arthritis. Light micrograph of a section through muscle affected by rheumatoid arthritis. Healthy muscle (pink fibres) is invaded by white blood cells (eosinophils and macrophages, dark spots) where degeneration is occuring. Rheumatoid arthritis is a condition where the patient's immune system attacks healthy tissue around their joints. Rheumatoid arthritis patients experience atrophy (loss) of muscle because of inflammation and limited mobility. Magnification unknown.