How can eating wheat hurt someone with celiac disease?

Damages intestine: There are very complex immune reactions between b-cells (antibody producing cells) and CD4 helper cells in celiac patients which result in anti-ttg antibody formation as well as the creation of gliaden sensitive cytotoxic t-cells which patrol the intestinal epithelium. These cytotoxic t-cells get stimulated by gluten to attack the intestinal lining cells and kill them resulting in tissue damage.

Tears them up: The simple answer is it tears them up.This is a food allergy/intolerance where exposure promotes a chronic battle between immune complexes & the intestine tissue.The injury can/does produce pain,failure to absorb nutrients/diarrhea or other symptoms.The symptoms are not as dramatic as some get with a peanut allergy, because it affects a different part of the immune system.