The human immune system needs to distinguish between foreign substances and the self. But how did it learn?

Medical researchers at the University of California, San Francisco have discovered that when a baby is developing in the womb, the mother's cells slip across the placenta, enter the fetus's body, and teach it to treat these cells as its own. 

In this way, the mother's cells help train the developing baby's cells to respond to outside threats but not overreact to harmless stimuli or the body's own tissues. 

The researchers demonstrated that the cells from the mother directly cause fetal tissue to produce more regulatory T cells, a particular type of immune cell.  "We found a specific mechanism for how the mother's cells induce the fetal immune system to be more tolerant," said [Jeff] Mold, who was the first author of the paper, which appeared in Science on Dec. 5.

The discovery could point the way to more successful transplants.

Link - via 3quarksdaily

From the Upcoming ueue, submitted by Marilyn Terrell.