The Hasselt Appendicitis Immunology and Environmental Study (HAPPIEST) aims at characterizing factors that influence the development and severity of acute appendicitis. In a cohort of 300 patients and 300 controls, environmental factors as well as genetic make-up of the innate immune system, focusing mainly on pattern recognition, will be analyzed in order to gain insight in their relative importance in the pathology of appendicitis. Furthermore, populations of micro-organisms present in the gut of patients will be characterized, and the interaction between relevant micro-organisms and the innate immune system will be analyzed.