First from my "nutritional" background. Breast milk contains white blood cells and antibodies (IgA, IgM and a little IgG). Along with these is an iron-grabbing protein, lactoferrin, which binds iron, keeping bacteria from getting the iron they need to grow on. Both white blood cells and lactoferrin have been shown to greatly reduce or curtail bacterial growth in freshly expressed breast milk. If the milk is frozen then this activity is greatly diminished or eliminated (depending on the freezer temp and length of time).
Colostrum contains a greater concentration of antibodies than the milk that follows it, however, the breast milk also contains these agents which have been shown to protect the infant agains life threatening intestinal disorders. Antibodies inactivate dangerous bacteria in the infant's digestive tract where they would cause great harm, while promoting the growth of "digestive friendly" bacteria, such as Lactobacillus.
Infants under the age of 4 to 6 months have an "open gut". This term refers to the interstitial space between the cells of the small intestine. In infants this allows the passage of intact proteins directly from their gut into their blood stream. It has been shown that the antibodies in breast milk will pass directly into the blood stream of infants--thus giving the infant passive immunities. These may be antibodies that are generated by the mother when the mother is exposed to some illness. These antibodies are therefore very specific in defending the infant against germs to which the mother (and therefore probably the infant) has been exposed.
Likewise, large proteins from other "foods" can pass into the blood stream of infants. This is one reason that if an infant is exposed to a food that may cause allergic reactions, this is the time that it will occur with the most devastating results, and usually life-long allergy. Milk proteins from cow's milk are often found whole in the blood stream of infants fed cow milk based formulas or whole cow's milk, and are more likely to develop allergies to them. Soy allergies, while not as common, can also occur. There is a link between the feeding of soy based formula and peanut allergies.
After 6 months, the gut "closes" and large proteins do not pass intact into the blood stream. This eliminates the passive immunity. However, there is still the benefit of decreased ear infections.
Infants that are breast fed are less likely to get ear infections. One of the reasons being that when the milk pools in the ear cannal (and this happens in all infants due to the shape of the eustacian tubes) with breast milk, there is less bacterial growth.
As proof of this I offer my first son, Kevin. He was born without an immune system. He was exclusively breastfed for the first 4 1/2 months of his life and healthy as anything. With the introduction of solids he broke out in "hives". With the removal of solids, and return to full breast feeding his skin cleared up--until 6 months of age, when he began to get one cold after another, ear infections, pneumonia. His skin became a nightmare of eczema sores. He was misserable. Food totally disagreed with his system. He was then diagnosed with Severe Combined Immune Deficiency (or bubble boy disease). He was given a bone marrow transplant at 9 months, however he became ill and the bone marrow "rejected" him, and he died following complications resultant from that at 18 months.
My breast milk kept him healthy for the first 6 months of his life. My breast milk kept him alive for 18 months, almost exclusively.
Resources:
For more information, visit the La Leche League website at http://www.lalecheleague.org/. I have been a proud member of LLL for several years.
Barret, James T. Textbook of Immunology, An introduction to immunochemistry and immunobiology. 3rd Ed. C.V. Mosby Company, St. Louis, 1978.
Hegarty, Vincent. Decisions in Nutrition. Times Mirror/Mosby College Publishing, St. Louis, 1988.
Whitney, Eleanor Noss & Hamilton, Eva May
Nunnelley. Understanding
Nutrition. 3rd Ed. West Publishing
Company, N.Y., 1984.