Saw this article. Not necessarily anything new here.
It still wasn't clear, however, what caused the weakened immunity in the first place. So Segal's team focused on cells called macrophages, the immune system's whistle-blowers. In people with Crohn's disease, they found that macrophages secrete lower levels of cytokines, the chemicals that rally other immune cells to infection sites (Journal of Experimental Medicine, DOI: 10.1084/jem.20091683).
The team concluded that ineffectual rallying of immune cells in people with defective macrophages is what allows intestinal bacteria to run amok in the early stages of an infection, setting in motion the series of events that leads to Crohn's disease.