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.