Complex sentence: Upon exposure to gluten, the immune system cross-reacts with the small-bowel tissue, causing an inflammatory reaction, which commonly leads to villous atrophy, which is shortening of the villi lining the small intestine.
Split into simpler sentences: Upon exposure to gluten the immune system cross-reacts with the small-bowel tissue, causing an inflammatory reaction. In some cases, that may lead to a shortening of the villi lining the small intestine.