Immune checkpoint inhibitors and the inflammatory dysregulation: decoding cutaneous and gastrointestinal immune-related adverse events for the allergist and immunologist

Immune checkpoint inhibitors (ICIs) targeting programmed cell death protein 1 (PD-1), its ligand (PD-L1), and cytotoxic T-lymphocyte-associated protein 4 (CTLA-4) have revolutionized oncology, yet they unleash a unique spectrum of inflammatory toxicities known as immune-related adverse events (irAEs). Cutaneous and gastrointestinal irAEs represent the most frequent and clinically challenging manifestations encountered by allergists and clinical immunologists. Unlike classical type I–IV hypersensitivity reactions, irAEs arise from disruption of peripheral immune tolerance, leading to T-cell hyperactivation, B-cell dysregulation with IgE and IgG4 skewing, and pro-inflammatory cytokine networks dominated by interleukin (IL)-6, IL-17, and interferon-gamma. IrAEs are best conceptualized as a distinct category of drug-induced autoimmunity, rather than a fifth type of hypersensitivity reaction. This review provides a comprehensive, clinically actionable framework for the practicing allergist. We dissect the divergent pathophysiology of anti-CTLA-4 versus anti-PD-1 toxicities, characterize the full spectrum of cutaneous eruptions and delineate the grading and management of ICI-induced colitis, which mimics inflammatory bowel disease but has distinct immune signatures. We critically evaluate emerging biomarkers (eosinophilia, serum IL-6, and fecal calprotectin) and present an evidence-based algorithm for step-up therapy: from topical corticosteroids and antihistamines for grade 1–2 rash, to systemic corticosteroids for grade 2–3 colitis, and biologic agents (infliximab, vedolizumab, tocilizumab, dupilumab) for corticosteroid-refractory cases. A dedicated section tackles the allergist’s most challenging dilemma (rechallenge after severe irAEs) providing evidence-based guidance. We identify critical knowledge gaps: pre-existing atopy as a risk factor, antihistamine prophylaxis utility, and overlapping toxicity management. This review repositions allergists as essential oncology partners, advancing irAE care from crisis response to precision immuno-toxicology.