Excellent - I have very solid textbook content. Now I have enough information from authoritative sources (Medical Physiology and Creasy & Resnik's Maternal-Fetal Medicine) to give a comprehensive 4-mark answer. Let me compose the full response.

Breast milk provides every nutrient a newborn requires in the right proportions - proteins, fats, carbohydrates, vitamins, and minerals - without any supplementation (provided the mother's own nutritional status is adequate). The calcium in breast milk is sufficient to meet the infant's needs for the rapid calcification of developing bones and teeth. The fat profile of breast milk is uniquely adapted for the infant's immature digestive system and rapidly growing brain.

Breast milk - particularly the early secretion called colostrum - is rich in secretory immunoglobulin A (IgA) antibodies directed against bacteria and viruses. It also contains macrophages and other immune cells. Because a newborn's own immune system is immature and cannot yet mount adequate responses, these maternally derived antibodies provide critical passive protection against infections during the most vulnerable period of life.

Breast milk contains specific factors (oligosaccharides and bioactive compounds) that promote the growth of beneficial lactobacilli in the infant's colon. These protective bacteria help guard against virulent pathogens such as Escherichia coli, reducing the risk of neonatal enterocolitis and diarrheal illness.

There is a documented dose-response relationship between the amount of human milk received and the degree of immunologic protection and developmental benefit the infant gains. For this reason, the WHO defines exclusive breastfeeding as giving only breast milk for the first 6 months - no other liquids or solids, not even water (exceptions: ORS, vitamin/mineral drops). The American Academy of Pediatrics (AAP) and ACOG both recommend exclusive breastfeeding for the first 6 months, as the evidence consistently shows superior outcomes in health, immunity, and neurodevelopment compared to formula or mixed feeding.