The pathology and causes of tissue eosinophilia in the gastrointestinal tract

Eosinophilic inflammation in the gastrointestinal tract may occur as a primary eosinophilic disorder or as a secondary response with other causes. Primary eosinophilic gastrointestinal disorders (EGIDs) are Th2‐mediated allergic diseases that overlap pathogenetically with atopic conditions involving other organs. The pathological diagnosis of primary EGIDs can be challenging, as the quantity of eosinophils considered to be ‘abnormal’ is difficult to define, and the diagnosis, by definition, requires exclusion of the far more common secondary causes. Our understanding of the basic biology and natural history of eosinophilic oesophagitis has advanced considerably over the last decade, whereas other EGIDs have proven more difficult to characterize; nonetheless, some recent advances have been made. This review summarizes current knowledge regarding the clinical presentation, diagnosis, natural history and treatment of EGIDs, including eosinophilic oesophagitis. We also draw attention to the numerous secondary causes of tissue eosinophilia in the gastrointestinal tract, and suggest a practical approach to the histological assessment, diagnosis and reporting of EGIDs.