Host immune dysfunction plays a crucial role in the onset, progression, and outcome of hemophagocytic lymphohistiocytosis (HLH). This study aimed to comprehensively evaluate the peripheral immune profiles in patients with newly diagnosed secondary hemophagocytic lymphohistiocytosis (sHLH), and explore their predictive value for patient prognosis. A total of 77 patients with sHLH were enrolled in this study, with 31 of them experiencing mortality. Flow cytometry was used to assess the percentages, absolute numbers, and phenotypes of lymphocyte subsets. Simultaneously, cytokine levels and routine laboratory indicators were also collected. In sHLH patients, lymphocyte subset absolute numbers were significantly impaired, accompanied by T cell hyperactivation, B cell hyperactivation, and increased plasmablast proliferation. Prognostic analysis revealed that lower CD8+ T cell percentages, elevated APTT, IL-6, IL-10 levels, and increased CD4+CD28null T cell proportions were associated with poor patient outcomes. The study demonstrates dysregulation in the counts and phenotypes of lymphocyte subsets in sHLH patients. Several key factors, including IL-6, IL-10, APTT, and various T cell percentages, have potential as prognostic markers and therapeutic targets in sHLH.

Hemophagocytic lymphohistiocytosis (HLH) is characterized by immune dysregulation with extensive inflammation and tissue destruction due to abnormal immune activation. Post-COVID-19 patients who have recovered with negative serologic tests may also present with secondary HLH, an unusual finding that our case demonstrates. A 73-year-old male with a notable past medical history of fall COVID-19 infection approximately 11 months prior presented initially to emergency services with a chief complaint of high fevers, lethargy, and progressive changes in mentation gradually progressive over the last 5 months\' duration. This presentation was concerning for HLH in view of the patient\'s high H score and clinical suspicion for HLH, and he was initiated on dexamethasone 20 mg daily. intravenous immune globulin (IVIG) protocol was also trialed, despite which the patient continued to deteriorate before expiring during the course of the hospitalization. sHLH following COVID-19 infection remains a poorly understood phenomenon. The severity of the COVID-19 infection does not appear to be related to one\'s predisposition to develop sHLH. The mortality of HLH remains high even with appropriate therapy.