A method of diagnosing, monitoring progression of, or monitoring treatment of inflammatory bowel disease comprises determining the levels of CD14+HLA-DRhi monocytes or monocytes expressing CCR7 or CCR9 or both CCR7 and CCR9 in a sample obtained from a subject, wherein high levels of CD14+HLA-DRhi monocytes or monocytes expressing CCR7 or CCR9 or both CCR7 and CCR9, or increased levels of CD14+HLA-DRhi monocytes or monocytes expressing CCR7 or CCR9 or both CCR7 and CCR9 compared to control, indicate the presence or progression of inflammatory bowel disease. Similar methods for diagnosing irritable bowel syndrome are also described. Various companion therapeutic methods and useful binding reagents are also described.