Lactose is a type of sugar found in milk. It cannot become absorbed by the body unless is gets changed into more simple sugars called glucose and galactose. This change happens when lactose passes through the stomach into the upper part of the gut (small intestine) and comes into contact with a protein (enzyme) called lactase.
This is a companion discussion topic for the original article at https://patient.info/allergies-blood-immune/food-allergy-and-intolerance/lactose-intolerance