In this context, pigment-grade TiO2 particles have been shown to stimulate secretion of IL-1β from macrophages isolated from mice carrying a mutation in the nucleotide-binding oligomerisation domain-containing 2 (NOD2) gene [129], a mutation that confers an increased risk for inflammatory bowel disease (IBD) [130, 131]. This evidence concerns the gene NOD2 and inflammatory bowel disease.