It has been shown that helminth parasites such as O. ostertagi drastically modulate their host environment. Changes in the abomasal pH rapidly occur during an infection and result in a reduction of pepsinogen activation, increased gastrin secretion and reduced bacteriostatic effect. The abomasal morphology is also drastically modified by the developing nematodes: mucous cell hyperplasia, epithelial cell damage, nodular appearance of parasitized glands and an accumulation of inflammatory cells such as eosinophils, neutrophils and lymphocytes. The parasite also modulates the host immune system so that it will take years of repeated exposure before a protective immune response is raised.