Milk is a white liquid food produced by the mammary glands of mammals and the caecilian Siphonops annulatus. It is the primary source of nutrition for young mammals before they are able to digest solid food. Immune factors and immune-modulating components in milk contribute to milk immunity. Early-lactation milk, which is called colostrum, contains antibodies that strengthen the immune system and thus reduce the risk of many diseases. Milk contains many nutrients, including calcium and protein, but also lactose and saturated fat.

Colostrum, or first milk, is the first form of milk produced by the mammary glands of humans and other mammals immediately following delivery of the newborn. It may be called beestings when referring to the first milk of a cow or similar animal. Most species will begin to generate colostrum just prior to giving birth. Colostrum has an especially high amount of bioactive compounds compared to mature milk to give the newborn the best possible start to life. Specifically, colostrum contains antibodies to protect the newborn against disease and infection, and immune and growth factors and other bioactives that help to activate a newborn's immune system, jumpstart gut function, and seed a healthy gut microbiome in the first few days of life. The bioactives found in colostrum are essential for a newborn's health, growth and vitality. Colostrum strengthens a baby's immune system and is filled with white blood cells to protect it from infection.