Milk is a nutrient-rich liquid food produced by the mammary glands of mammals. It is the primary source of nutrition for young mammals, including breastfed human infants before they are able to digest solid food. Early-lactation milk is called colostrum, which contains antibodies that strengthen the immune system and thus reduces the risk of many diseases. It holds many other nutrients, including protein and lactose. Interspecies consumption of milk is not uncommon, particularly among humans, many of whom consume the milk of other mammals.