Table 1.

The challenges created by the illusion of similarity

The paradox	Mouse	Human
Fibrosis and IBD	Fibrotic lesions such as those seen in CD are rarely observed in mouse models of IBD	Fibrosis and strictures are a common occurrence in CD
IL-17 and IBD	IL-17 has been shown to be deleterious or protective	Antibody to IL-17 exacerbates inflammatory bowel disease
Phenotype of immune cells	T cells from mice exposed to conventional microbiota resemble T cells from neonatal humans	Mature T cell phenotype in humans observed only in mice colonized with complex microbiota
Clinical signs of infections	Mice fail to develop the clinical signs seen in humans after infection with H. pylori, C. difficile or influenza virus	H. pylori, C. difficile and influenza virus each produce a distinct disease in humans
LPS	Mice relatively resistant to LPS	Humans very sensitive to LPS

In these representative examples of differences between animal models and human disease, some heterogeneity could reflect technical limitations of genetic deletions that might not mimic specific alleles, whereas others might reflect environmental differences that affect microbiota or fundamental differences with evolutionary roots. H. pylori, Helicobacter pylori; C. difficile, Clostridium difficile; LPS, lipopolysaccharide.