here is what i found, on Wikipedia
THIS IS WHAT COLOSTRUM IS FOR:
Under the influence of the hormones prolactin and oxytocin, women produce milk after childbirth to feed the baby. The initial milk produced is often referred to as colostrum, which is high in the immunoglobulin IgA, which coats the gastrointestinal tract. This helps to protect the newborn until its own immune system is functioning properly, and creates a mild laxative effect, expelling meconium and helping to prevent the build up of bilirubin (a contributory factor in jaundice).
Now as for the Composition of Breast Milk:
The exact integrated properties of breast milk are not entirely understood, but the nutrient content after this period is relatively consistent and draws its ingredients from the mother's food supply. If that supply is found lacking, content is obtained from the mother's bodily stores. The exact composition of breast milk varies from day to day, depending on food consumption and environment, meaning that the ratio of water to fat fluctuates. Foremilk, the milk released at the beginning of a feed, is watery, low in fat and high in carbohydrates relative to the creamier hindmilk which is released as the feed progresses. The breast can never be truly "emptied" since milk production is a continuous biological process.
Human milk contains 0.8% to 0.9% protein, 10% to 15% fat, 6.9% to 7.2% carbohydrates and 0.2% ash (minerals). Carbohydrates are mainly lactose; several lactose-based oligosaccharides have been identified as minor components. The principal proteins are casein homologous to bovine beta-casein, alpha-lactalbumin, lactoferrin, IgA, lysozyme and serum albumin. Non-protein nitrogen-containing compounds, making up 25% of the milk's nitrogen, include urea, uric acid, creatine, creatinine, amino acids and nucleotides. Breast milk has circadian variations; some of the nucleotides have acrophases during the night, others during the day.
Mother's milk has been shown to supply a type of endocannabinoid (the natural neurotransmitters which marijuana simulates), 2-Arachidonoyl glycerol.
Though it now is almost universally prescribed, in some countries in the 1950s the practice of breastfeeding went through a period where it was out of vogue and the use of infant formula was considered superior to breast milk. However, it is now universally recognized that there is no commercial formula that can equal breast milk. In addition to the appropriate amounts of carbohydrate, protein and fat, breast milk also provides vitamins, minerals, digestive enzymes and hormones - all of the things that a growing infant will require. Breast milk also contains antibodies and lymphocytes from the mother that help the baby resist infections. The immune function of breastmilk is individualized, as the mother, through her touching and taking care of the baby, comes into contact with pathogens that colonize the baby and consequently her body makes the appropriate antibodies and immune cells.
but thats my .2c's...lol