In response to all the hubbub from the NYT article and the federal government reporting that formula feeding is "risky," Slate has published a fascinating and accessible article on the immunology of breastfeeding. Apparently, most doctors have it wrong, humans do not absorb antibodies into their bloodstream through nursing as other mammals do. Apparently humans and other primates have a dedicated transport system for antibodies from the maternal supply.

When you ask a bunch of doctors about how breast-feeding prevents infection, they get it wrong—I know they do, because I've asked the question. Doctors tell you that colostrum (produced in the first three days or so after a baby is born) and breast milk are full of maternal antibodies. Next, doctors say that these maternal antibodies are absorbed into the infant's blood circulation and thus serve to protect infants from disease.

That's the correct description of the immunology of breast-feeding for most mammals. It's also true that human colostrum and milk are rich in maternal antibodies—colostrum is pretty much antibody soup. And babies take in these antibodies as they nurse. But human babies are never able to absorb maternal antibodies from milk or colostrum into the bloodstream, except perhaps in the minutest amounts. Maternal antibodies in milk and colostrum protect against infection—but only locally, working inside the baby's gastrointestinal tract.

This information will surprise farmers, veterinarians, and strongly invested proponents of breast-feeding. After all, if a newborn piglet is deprived of its mother's colostrum for the first eight hours of life, it is almost guaranteed to become sick and die. Similarly, newborn horses, cats, dogs, and most other mammals are not likely to survive long if they are deprived of colostrum. The reason is simple: Most mammals are born without any antibodies, or only the tiniest amounts, circulating in their blood. That leaves them defenseless at birth against viruses, bacteria, and other pathogens. Fortunately, for a brief period after birth, the antibody molecules in colostrum can easily pass through the bowel walls of babies of each of these species.

But human newborns, it turns out, differ from most other mammals in the way they acquire maternal antibodies. (Before the creationists get too excited, I should point out that everything I am about to say applies to monkeys as well as to people.) Newborn infants get their maternal protection before birth, via an active transport system in the human placenta that carries maternal antibodies from the mother to the fetus. Unlike all those other animals, human babies are born with all the maternal antibodies they will ever have. That's why we don't need to absorb maternal antibodies from colostrum. And it's why formula-fed babies are not at a disadvantage, compared with breast-fed babies, in their supply of circulating maternal antibodies.

None of this is my discovery. It was well-known, even commonplace, in the immunological literature of 40 years ago. But as the field turned to other matters, these findings just sort of fell out of fashion (though I've certainly come upon plenty of modern papers whose authors understand the idea). Because of the modern aversion to looking at older research, a surprisingly large number of doctors, especially nonimmunologists, have either forgotten this aspect of human immunity or never knew about it.

He's right about that older research thing. If it was discovered before about 1990 (and doesn't have an abstract accessible to pubmed) most scientists will never know about it. It's just not efficient anymore to parse through decades worth of journals that aren't electronically catalogued when there might be a nice electronically available review article posted online.