Abstract
Rituximab (RTX), a widely used monoclonal antibody for hematologic malignancies and rheumatologic disorders, is known for infusion-related reactions. However, its potential to induce colitis is often overlooked. We present an 85-year-old woman with chronic lymphocytic leukemia experiencing severe adverse effects during her fourth RTX infusion, including abdominal pain, hypotension, and bright red blood per rectum. Computed tomography of the abdomen and pelvis with contrast revealed pancolonic mural wall thickening without perforation. Prompt treatment with vasopressors and intravenous fluids led to symptom resolution within 24 hours. We highlighted the importance of recognizing RTX-induced colitis and discussed and depicted immunological dysregulation mechanisms involved.