In simple language
: Human milk provides virtually all the protein, sugar, and fat your baby needs to be healthy, and it also contains many substances that benefit your baby’s immune system, including antibodies, immune factors, enzymes, and white blood cells. These substances protect your baby against a wide variety of diseases and infections not only while he is breastfeeding. If you're breastfeeding, kissing your baby’s head like you often do whilst in a carrier, will mean that you pick up anything they’ve been in contact with in their environment and your milk will adapt to help fight that
: breastfeeding protects against infections through specific and non-specific immune factors and "has long-term consequences for metabolism and disease later in life"