This review explores the role of T helper 9 (Th9) cells in allergic diseases and how the microbiota influences Th9 activity. Certain microbes, such as Staphylococcus aureus and Candida albicans, can stimulate Th9 responses, exacerbating allergic inflammation, while microbial and dietary metabolites like retinoic acid, butyrate, and vitamin D may
