Light micrograph of a longitudinal section through the human appendix showing acute appendicitis. Acute inflammation of the appendix is one of the most common surgical emergencies in children and young adults. The earliest change is ulceration of the mucosa (centre); the glandular layer is replaced by an inflammatory exudate and pus enters the lumen (interior) at left. As the condition progresses, the inflammation spreads through all layers of the wall of the appendix with the eventual formation of exudate on the peritoneal surface at right. Subsequent inflammation of the peritoneum (peritonitis) is responsible for the classical clinical features of appendicitis.