Gut microbiota comprise important environmental exposures that influence human immune systems and may alter the clinical course of inflammatory bowel disease (IBD). Little is known about the role of gut bacteriophages (viral components that infect prokaryotic bacteria) and their interactions with the host’s immune responses. We tested the hypotheses that (1) immune responses of individuals with IBD to phages differ from those without IBD and (2) immune responses to phages are associated with disease type (i.e.