Paradoxical inflammatory switches during biologic therapy: a mechanistic framework, clinical algorithm, and pediatric illustrations

Paradoxical inflammatory reactions, also referred to as flip–flop phenomena, are increasingly recognized complications of biologic therapies targeting specific immune pathways. These reactions are characterized by the emergence of a new inflammatory phenotype with opposing immunologic polarization or by unexpected exacerbation of the underlying disease despite prior therapeutic response. Although numerous case reports have been published, a unified mechanistic framework and a practical clinical approach remain lacking, particularly in pediatric populations. In this narrative review, we synthesize current evidence on the immunopathogenesis of flip–flop reactions, focusing on dynamic interactions between the Th1/Th17, Th2, and interferon–JAK/STAT axes. We propose that paradoxical inflammation can represent a mechanistically consistent consequence of selective immune pressure rather than a coincidental adverse event. Five illustrative pediatric cases are presented as proof of concept, demonstrating distinct pathways leading to eczematous, psoriasiform, and interferon-driven phenotypes during biologic therapy. Based on mechanistic insights and clinical experience, we introduce a practical diagnostic and therapeutic algorithm designed to support early recognition and severity-adapted management of flip–flop reactions. Recognition of immune rebalancing as a therapeutic goal may facilitate rational treatment selection and improve outcomes in both pediatric and adult patients receiving biologic agents.