Acute appendicitis. Light micrograph through a section of an appendix showing acute appendicitis. Here, the epithelial lining of the appendix (lower edge of the white region) has become ulcerated (right) and is secreting an inflammatory exudate (thick fluid, top right). White blood cells (small, dark purple) are present in high numbers. As the disease progresses, the inflammation will spread until until the exudate is present on the external surface (peritoneum) of the appendix. Inflammation of the peritoneum (peritonitis) is serious and often requires emergency surgery.