Why do some immune systems reject cow's milk protein while others tolerate it? Around 5 percent of young people in the UK show allergy to cow's milk. Some become sensitized even through breast milk when the mother drinks cow's milk. Researchers used flow cytometry to study immune cells in the intestine. They compared two types of lymphocytes (immune cells). One type triggers allergic reactions. The other type calms the response. The key finding was a shortage of regulatory cells called TH3 cells. These cells produce a protein called TGF-beta that quiets nearby immune activity. Without enough TH3 cells, the immune system cannot build tolerance. Any cell that reacts to food protein keeps responding unchecked.