Human milk is more than food. It's a complex living substance, like blood, with a long list of active germ-fighting and health-promoting ingredients. These help protect babies against all kinds of infections, common and not-so-common.

A drop of breastmilk contains around one million white blood cells. These cells, called macrophages ("big eaters"), gobble up germs. Breastmilk is also power-packed with immunoglobulin A (IgA), which coats the lining of babies' immature intestines, preventing germs from leaking through. Secretory IgA also works to prevent food allergies. By coating the intestinal lining like a protective paint, it prevents molecules of foreign foods from getting into the bloodstream to set up an allergic reaction.

Colostrum, the milk mothers produce in the first few days after birth, is especially rich in IgA, just at the time when the newborn is first exposed to the outside world and needs protection from germs and foreign substances entering his body. Colostrum also contains higher amounts of white blood cells and infection-fighting substances than mature milk. Think of colostrum as your baby's first important immunization.

As babies grow, mother's milk continues to provide important protection against infection and disease. Human infants receive antibodies through the placenta, but these are gradually used up during the first six months. Human milk fills in the immunity gap until baby's own immune system matures and kicks in. Even babies who continue to nurse into toddlerhood benefit from the many immune factors in their mother's milk.

Immunities made-to-order. Each mother provides custom-designed milk to protect her infant. When a baby is exposed to a new germ, mother's body manufactures antibodies to that germ. These antibodies show up in her milk and are passed along to her baby. Many a nursing mother can tell the story of the entire family--dad, mom, siblings--coming down with the flu and the nursing baby having the mildest case, or not getting sick at all. When mother comes down with a bug, the best thing she can do for her baby is to keep breastfeeding.

Derrick and Patrice Jelliffe, pioneers in breastfeeding research, stated that breastfed infants are "biochemically different." This difference in body chemistry may be the reason they are healthier. While babies are breastfeeding, they have fewer and less serious respiratory infections, less diarrhea, and less vomiting. When breastfed babies do become ill, they are less likely to become dehydrated and need hospitalization.

Here are some specific ways in which breastfeeding protects babies from illness:

• Friendly to little ears. Ear infections are a childhood nuisance, often following on the heels of stuffy noses and colds. The middle ear fills with fluid, and eventually that fluid becomes infected, causing pain, especially in the middle of the night. Repeated ear infections, or those that go untreated, can lead to hearing loss. This is an important concern in young children, since hearing difficulties can interfere with language, and language problems can later affect reading skills.

  Breastfeeding protects against ear infections in four possible ways:

  1. The many germ-fighting ingredients in human milk keep harmful bacteria from bothering baby, so that stuffed-up noses and ears are less likely to become infected middle ears.
  2. Because breastfed babies are fed in a more upright position, they're less likely to experience milk backing up through the eustachian tube into their ears; if this does happen during a breastfeeding session, human milk is less irritating to the tissues of the middle ear than infant formula.
  3. Breastfed babies have fewer, or at least less severe, colds than formula-fed babies. Fewer colds means fewer ear infections.
  4. Breastfed babies have fewer respiratory allergies, another cause of fluid building up in the middle ear, which setts the stage for bacteria to grow.

• Protects tiny tummies. Human milk excels at protecting babies from diarrhea and tummy upsets. This is important not only for individual babies but also on a global scale. Diarrhea is a leading cause of infant mortality worldwide, and breastfeeding is the simplest, most cost- effective way to protect babies from repeated bouts of gastrointestinal illness.

  Another way in which breastfeeding protects tiny tummies is by promoting the growth of healthful bacteria in the intestines. Intestines are healthiest when you can keep the right "bugs" in the bowels. The healthful bacteria, known as bifidus bacteria, do good things for the body in return for a warm place to live. They manufacture vitamins and nutrients and keep the harmful bacteria in check. The high levels of lactose in breastmilk particularly encourage the growth of the healthful resident bacteria Lactobacillus bifidus.

• Protects against other infection. Studies have found that breastfeeding protects against a wide variety of other diseases. Here's a partial list:
  • Haemophilus influenzae type B
  • Pneumonia caused by Streptococcus pneumoniae
  • Meningitis
  • Infant botulism
  • Urinary tract infections
  • Cholera
  • Salmonella
  • E. coli infections
  • Respiratory syncytial virus

• Reduces risk of SIDS. Many parents are relieved to learn that breastfed babies are less likely to become victims of SIDS. There are many ways in which breastfeeding could influence the incidence of SIDS. One recent theory suggests that infants who die of SIDS may sleep too deeply and fail to awaken if they stop breathing for a moment or two, as babies often do when they're sleeping. Breastfed babies sleep less deeply and thus may be more likely to wake up if there is a problem with their breathing. Breastfeeding's protection against infection may also help to lower the SIDS risk. (See Breastfeeding and SIDS)

• Fewer problems with reflux. While all babies spit up a bit, some regurgitate excessive amounts of milk, because of a condition called gastroesophageal reflux (GER). Normally, the circular band of muscle where the esophagus joins the stomach acts like a one-way valve, keeping milk, food, and stomach acids from backing up into the esophagus when the stomach contracts. When it doesn't do its job and these acids enter the esophagus, the result is an irritation that adults would call heartburn. In many infants, it takes six months to a year for this muscle to mature enough to prevent this regurgitation or reflux. GER is less of a problem in breastfed infants because breastmilk is emptied twice as fast from the stomach. It's less likely to be regurgitated than slow-to-digest formula with its tough casein curds.