Appendicitis. Light micrograph through a section of an appendix showing appendicitis. Appendicitis is an inflammation of the appendix, an organ attached to the large intestine that appears to have no function. Here, the epithelial lining (mucosa, purple edges) of the appendix is ulcerated, producing an inflammatory exudate (a thick fluid that is secreted as a result of disease). As the disease progresses, the inflammation will increase as it spreads throughout the appendix until exudate is present on all surfaces. If it spreads to the peritoneum (the lining of the abdominal cavity) immediately surrounding the appendix, the condition becomes critical and may be fatal if left untreated. Treatment is by surgical removal with intravenous administration of antibiotics.