During gestation, the placenta of the cow effectively separates the blood of the fetus from that of the dam and prevents any transfer of protective immunity while in the uterus. Therefore, the calf is born completely dependent on the absorption of maternal antibodies from colostrum after birth. Colostrum is the milk produced from the mammary gland in the first 24 hours after birth. A calf's gastrointestinal tract is designed to temporarily allow the absorption of large molecules including antibodies from the small intestine, but only during the first 24 hours after birth. Although colostrum contains several different types of immunoglobulins, IgG accounts for roughly 85 percent of the total volume. IgG absorption is most efficient in the first four hours of life and declines rapidly after 12 hours of age. At 24 hours, the gut is completely closed and there is no further immunoglobulin absorption. These absorbed antibodies must be consumed in order to protect the calf from disease organisms until its own immune system becomes functional.