IL10 and isolated hyperferritinemia: R-CHOP immunochemotherapy achieved rapid symptom resolution and complete metabolic response on follow-up PET-CT.<h4>Conclusion</h4>The triad of extreme hyperferritinemia, elevated LDH, and markedly increased IL-10 may serve as a useful diagnostic clue to raise suspicion for IVLBCL in FUO patients, though its interpretation is confounded by secondary HLH and requires prospective validation.