Colostrum (known colloquially as beestings, bisnings or first milk) is the first form of milk produced by the mammary glands of mammals (including humans) immediately following delivery of the newborn. 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.

At birth, the surroundings of the newborn mammal change from the relatively sterile environment in the mother’s uterus, with a constant nutrient supply via the placenta, to the microbe rich environment outside with irregular oral intake of complex milk nutrients through the gastrointestinal tract. This transition puts high demands on the gastrointestinal tract of the neonate, as the gut plays an important part in both the digestive system and the immune system. Colostrum has evolved to care for highly sensitive mammalian neonates and contributes significantly to initial immunological defense as well as to the growth, development, and maturation of the neonate’s gastrointestinal tract by providing key nutrients and bioactive factors.

Colostrum also has a mild laxative effect, encouraging the passing of the baby's first stool, which is called meconium. This clears excess bilirubin, a waste-product of dead red blood cells, which is produced in large quantities at birth due to blood volume reduction from the infant's body and helps prevent jaundice.

 

Note:   The above text is excerpted from the Wikipedia article Colostrum, which has been released under the GNU Free Documentation License.