Interleukin-6: A Central Biomarker in Adult and Pediatric Cancer and Infectious Disease

Interleukin-6 (IL-6) is a multifunctional cytokine with an essential role in immunity, inflammation, and cancer. Produced by immune, stromal and epithelial cells in response to infection or tissue stress, IL-6 regulates immune responses, acute-phase proteins (including serum amyloid A and C-reactive protein), hematopoiesis, and tissue remodeling. These effects are mediated via classical and trans-signaling pathways, which activate key intracellular cascades such as JAK/STAT3, MAPK, and PI3K/AKT. Accumulating evidence implicates dysregulated IL-6 signaling in both oncologic and infectious diseases, where it contributes to disease progression, immune evasion, and therapeutic resistance. This review aims to critically examine the role of IL-6 as a biomarker and therapeutic target in these two major clinical contexts: in cancer, IL-6 levels reflect tumor burden, prognosis, and therapy resistance in both adult and pediatric patients; in infectious diseases, circulating IL-6 may support early diagnosis and risk stratification, particularly in vulnerable pediatric populations. By integrating molecular mechanisms with clinical evidence, this review highlights IL-6 as a unifying biomarker linking inflammation, infection, and malignancy. It also addresses current limitations, including assay variability, lack of standardized reference ranges, especially in children, and challenges in clinical implementation.