TNF and Alzheimer disease: To explain the relationship between AD severity, relative abundance of gut microbiota, and metabolite alterations, we performed the Spearman rank correlation analysis on AD-related conditions like ear swelling, ear thickness, skin thickness, mast cell counting, TEWL, expression levels of TNF-α, IFN-γ and IgE, relative abundance of gut microbiota, metabolites and SCFAs (Figure 12).