“…In comparison to conventionally housed animals, GF animals have decreased plasma cells and IgA, decreased expression of activation markers on intestinal macrophages, decreased MHCII on epithelial cells, decreased nitric oxide, and histamine levels in the small intestine (Gordon, 1959; Beaver and Wostmann, 1962; Glaister, 1973; Moreau et al, 1978; Matsumoto et al, 1992; Mikkelsen et al, 2004; Sobko et al, 2004; Smith et al, 2007). Peyer’s patch follicles are reduced in number and size and the mesenteric lymph nodes are smaller, less cellular, and do not have germinal centers in GF animals (Gordon, 1959; Glaister, 1973). However, reconstitution of GF mice with an intestinal microflora is sufficient to restore the mucosal immune system (Umesaki et al, 1995).…”