Not all cheese is made with animal-derived rennet. There are a number of rennetless cheeses whose coagulating enzymes are vegetable, microbial, or genetically engineered. One group of rennetless cheeses has acidic levels high enough not to require enzymes for coagulation. This group includes cottage cheese, ricotta, and some varieties of mozzarella. Rennetless has also become a generic term for any cheese made without any animal derived enzymes.
"Traditionally, the first step in making cheese was to kill a newly-born, milk-fed calf and remove its stomach to make rennet. The rennet was derived from the inner lining of the abomasum, the fourth stomach of the calf or any other animal classified as a ruminant. " คัดลอกมาจาก http://www.vegparadise.com/news32.html