Its not an allergy. An allergy is when the immune system reacts inappropriately. Lactose intolerance is just that a person cant digest lactose, usually because they lack the enzyme that does this. Normally a mammal will lose this enzyme after they are weaned but due to a mutation, many humans retain this. Lactose intolerant people do not.
It is when your body cannot digest lactose which is the sugars in milk. It varies from a milk allergy, as a true milk allergy is when someone is allergic to the protein in milk. People who are lactose intolerant will usually have bad cramps and diarhea or even constipation in some cases.
What happens in the body when lactose is consumed is that lactase an enzyme breaks lactose down into galactose and glucose, both simple sugars the body can use. With lactose intolerance te person lacks that enzyme causing a build up of lactose which cause digestive problems
Most humans loose the ability to digest lactose. The longer a population has consumed dairy the more common the gene mutation is. Most Caucasians have the mutation while most Asians and Africans don't.
When your allerigic to dairy products (ice cream, milk, cheese ect..) and you get really bad stomach aches and leads to not so pleasant bathroom trips..
Mostly its People who have lactose intolerance have trouble digesting lactose, a type of sugar found in milk and other dairy foods.