Acute interstitial nephritis (AIN) describes a lesion with interstitial edema with a mononuclear cell infiltrate invading the interstitium and tubules, composed mostly of T lymphocytes. There are scattered plasma cells and macrophages, occasionally with non-necrotizing granulomas, and variable presence of eosinophils. When inflammation is due to infection, the lesion is diagnosed according to the specific pathogen involved (eg, bacterial, viral, or fungal). AIN commonly manifests clinically as acute kidney injury, with rapid decline in glomerular filtration rate, often with fever, eosinophilia, hematuria, sterile pyuria, and non-nephrotic proteinuria.