How are IBS and IBD related?

"Is Irritable Bowel Syndrome a Low-Grade Inflammatory Bowel Disease?" in Gastroenterol Clin N. Am (2005)
"The authors believe that IBD and at least a subset of IBS patients exist at two ends of the same spectrum of pathophysiology, which involves immune activation and inflammation. This is prompted by the observation that IBS symptoms may precede IBD, which reflects gut dysfunction generated by subclinical inflammation. It also has been observed that IBS occurs in patients in remission from IBD. This concept is underpinned by results of basic scientific studies in animal models showing that immune activation and inflammation restricted to the mucosal compartment result in profound changes in neuromuscular function that may persist after recovery of the mucosa. Emerging evidence shows similarities in genotype between IBD and a subset of IBS patients; polymorphisms of genes that encode cytokine secretion may result in an imbalance of pro- and counter-inflammatory signals. This in turn would lead to inefficient down-regulation of inflammatory responses and promote low-grade inflammation. It is a matter of the severity of inflammation that separates IBD and this IBS subset, and this may reflect additional genetic abnormalities or greater exposure to environmental factors in the case of IBD. This prompts the question as to whether IBD is more common in patients with IBS, and there is some evidence to support this. Clearly in IBD the brunt of immune-mediated injury is borne by the mucosal compartment, whereas in IBS, the mucosal compartment may play a role in initiating events, but the brunt of injury is taken by the deeper neuromuscular tissues. Further work is required to elucidate differences in the regulation of immune activity between these compartments to better understand the relationship of IBD and IBS."