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IL‑33 is an epithelial‑derived “alarmin” cytokine released from stressed or damaged barrier tissues, including keratinocytes in the skin. It belongs to the IL‑1 family and signals primarily through the ST2 (IL‑1RL1) receptor on mast cells, group 2 innate lymphoid cells (ILC2s), Th2 cells, and other immune effectors, rapidly amplifying type 2 immune responses.

In skin, IL‑33 sits upstream of the classic Th2 cytokines IL‑4 and IL‑13. When barrier integrity is disturbed – mechanical damage, irritants, or tight junction failure – keratinocytes release IL‑33, which promotes IL‑4/IL‑13 production and eosinophilic inflammation. This places IL‑33 at the initiating end of the barrier–inflammation loop rather than as a downstream consequence of established Th2 activity.

IL‑33 also has a direct barrier effect independent of IL‑4 and IL‑13. Through ERK/STAT3 signalling in keratinocytes, it downregulates claudin‑1 ( CLDN1), weakening tight junction integrity and increasing paracellular permeability. This means IL‑33 contributes to barrier dysfunction in two ways simultaneously: indirectly by driving IL‑4/IL‑13 release, and directly by suppressing the tight junction protein that seals the stratum granulosum.

Genetically and clinically, IL‑33 is linked to atopic conditions across organs. Variants in IL33 or its receptor ST2 associate with atopic dermatitis and asthma, reflecting its role as a shared upstream driver of type 2 inflammation in skin, airway, and gut epithelium.