Final answer:
First milk, or colostrum, is rich in immunoglobulins, a type of protein crucial for the newborn's immune system. The composition of breast milk changes during a feeding from thirst-quenching foremilk to appetite-satisfying hindmilk, which is rich in fat.
Step-by-step explanation:
That first milk, known as colostrum, contains immunoglobulins which are classified as proteins. In the initial days of lactation, colostrum plays a vital role in enhancing the newborn's immune defenses. As lactation progresses, the composition of breast milk undergoes changes in a single feeding from foremilk to hindmilk.
The foremilk is watery and translucent, rich in lactose and protein, ideal for quenching the infant's thirst. Towards the end of a feeding, the milk that is secreted is known as hindmilk. This is opaque and creamy, high in fat content, which helps to satisfy the infant's appetite.