A White man in his 70s with a medical history notable only for mitral valve repair was incidentally found to have cervical lymphadenopathy on magnetic resonance imaging while undergoing workup for shoulder pain. A fine-needle biopsy of an enlarged left-sided cervical lymph node yielded benign results. One month later, he presented to a local emergency department with diffuse abdominal pain. Symptoms began 6 weeks earlier and had been intermittent in nature. He had also experienced fatigue, night sweats, decreased appetite, and weight loss for 6 weeks. He otherwise denied having fevers, myalgias, nausea, vomiting, diarrhea, or easy bruising. Physical examination revealed bilateral cervical, axillary, and inguinal lymphadenopathy in addition to mild diffuse abdominal pain. Laboratory results revealed a hemoglobin level of 13.1 g/dL (to convert to g/L, multiply by 10), a white blood cell count of 6.4 × 103/μL, and a platelet count of 404 × 103/μL (to convert to × 109/L, multiply by 1) in addition to negative results for HIV, hepatitis B virus, and hepatitis C assays. Additional workup results were notable for elevated levels of interleukin (IL) 10 (1091 pg/mL), IL-6 (3.1 pg/mL), C-reactive protein (4.9 mg/dL [to convert to mg/L, multiply by 10]), and erythrocyte sedimentation rate (79 mm/h). A computed tomography scan of the neck, chest, abdomen, and pelvis further revealed extensive cervical, supraclavicular, retroperitoneal, pelvic, and inguinal lymphadenopathy, with extensive inflammation and fat stranding adjacent to these enlarged nodes. Positron emission tomography–computed tomography further demonstrated fluorodeoxyglucose-avid lymph nodes above and below the diaphragm. The patient then underwent excisional biopsy of an enlarged right cervical lymph node (Figure).