Final answer:
Colostrum is the clear, yellow, early product of breast milk that is high in secretory antibodies, helping to establish initial immune defenses in the newborn during the first few days postpartum.
Step-by-step explanation:
The clear, yellow, early product of breast milk that is very high in secretory antibodies is called colostrum. This substance is produced during the first 48-72 hours postpartum, and it is instrumental in providing essential nutrients and immunoglobulins to the newborn, which aid in gastrointestinal and systemic immunity. Colostrum is particularly rich in proteins and contains less fat and glucose than mature breast milk. While associated hormones like prolactin and oxytocin play key roles in lactation, it is colostrum that is the initial milk containing immunoglobulins vital for the newborn's immune defenses. The lactating mother supplies adequate hydration and nutrients through colostrum and breast milk in the first months of the infant's life.