I’d like to preliminarily state that medical advice is not always the best. It is always best t do your own research to confirm anything you hear even if it comes from a physician! Think and act scientifically! I say this because for most of the 20th century physicians supported formula feeding and had no idea that this led to extreme infant morbidity. By the 1940’s physicians regarded formula as “a well known, popular, and safe substitute for breastmilk.” And that as a result of this, “by 1946–1950, initial breast-feeding of first-born infants had decreased to 50% and only 20% were breast-fed for at least 6 months.”
Breastfeeding your baby provides it with its only source of IgA, a specific secretory antibody isotype that is necessary for protection of mucosal surfaces. Breastfeeding transfers 0.25-0.5 grams of IgA from mother to baby per day. This is critical in the time period of 3 months to a year, where the baby experiences diminishing maternally transmitted antibodies and has yet to produce significant amounts of their own antibody: this time period is where many infant deaths occur due to a vulnerable immune system (see chart). The main benefit provided directly by this is a greatly lowered rate of infection of the baby due to a strengthened immune response in the GALT and MALT (Gut and mucosal associated lymphoid tissue) to fight disease before it can breach the mucosal membranes and cause systemic infection. This IgA protection has been directly proven to reduce the prevalence and numbers of Clostridium difficile and Escherichia coli in the gut microbiota even in health, and the occurrence of infection by a wide range of dangerous pathogens such as Helicobacter pylori, Giardia lamblia, and Salmonella. This reduction accounts for less tonsillitis, sepsis and necrotizing enterocolitis (which is twenty times more common in formula fed babies) occurrence in breastfed babies for years.
All of these immune system responses are aided by other elements of the colostrum, such as lactoferrin and other immune-stimulatory and modulatory cytokines, peptides, and oligosaccharides. These, as well as growth factors such as PDGF and TGF a/b cause the thymus in breastfed babies to be larger than in formula-fed children, indicating higher levels of T-cell production. T cells are a lymphocyte (white-blood cell) absolutely central to the adaptive response, which is recruited when the innate immune response cannot clear a pathogen. The innate immune response is necessary, as mutants with abnormalities in T-cells, B-cells, or other lymphoid associated cells or signals typically are lethal.
Not only does the more advances immune system in breastfed babies protect against pathogens, but also cancers. One study found that as the duration of breastfeeding decreased, a child’s odds ratio for cancer development increased for all types of cancers. Another study considering 99 different childhood cancers found support for the hypothesis that breastfeeding reduces cancer risk and found especially high statistical support (p<0.001) for exclusive breastfeeding (not combined with formula feeding) being tied to childhood cancer risk (the control groups averaged 4.6 months while the cancer groups averaged 3.2 months of exclusive breastfeeding).
Immunological benefits of breastfeeding, since they affect the development of the immune system and therefore can cause prolonged immune system strength by increasing lymphocyte production, etc. This is proven in a study tracking the healthcare of breastfed vs purely formula fed babies, finding the babies that were 1,000 exclusively breastfed for at least 3 months required 2,033 less office visits, 212 less days of hospitalization, and 609 fewer prescriptions compared to 1,000 formula fed babies.