MILR1 (mast cell immunoglobulin-like receptor 1), also known as Allergin-1, is an inhibitory immunoreceptor that negatively regulates mast cell and basophil activation. The receptor suppresses IgE-mediated degranulation and FcεRI-dependent signaling 1, thereby inhibiting type I immediate hypersensitivity reactions. MILR1 expression levels are regulated by promoter polymorphisms; the C allele of rs6504230 increases MILR1 expression and confers protective effects against atopy by reducing susceptibility to allergen-specific IgE production 2. At the mechanistic level, MILR1 functions as a transmembrane signaling receptor on cell surfaces that inhibits basophil-mediated anaphylaxis following oral allergen exposure 1. Notably, MILR1 does not regulate neutrophilic inflammation in respiratory viral infection models 3. Clinically, MILR1 serves as a biomarker across multiple disease contexts. Elevated serum MILR1 levels are associated with severe cytokine release syndrome in CAR T-cell therapy patients 4 and COVID-19 disease severity 5. Additionally, MILR1 alterations correlate with immune dysregulation in common variable immunodeficiency 6 and depression in Parkinson's disease patients 7. These findings suggest MILR1 as both a mechanistic target for allergic disease intervention and a potential clinical biomarker for immune dysregulation.