It is not every week that a major tenet of biology is overturned. Immunologists are now being forced to abandon the idea that the immune systems of newborn mammals cannnot attack invading pathogens. The discovery is even prompting a rethink of the idea that the immune system works by distinguishing "self" from "nonself". A trio of papers published in last week's issue of Science (vol 271, pp 1723, 1726 and 1728) show that the immune cells of young mammals are primed to fight invaders right from the moment of birth. Immunologists had argued that a newborn's immune cells initially tolerate any molecules they encounter. The idea was that the immune system used this period to learn which molecules are found in the body so that it does not attack them later on.
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