Every human body contains a small population of cells that are not genetically its own -- cells that crossed the placenta during pregnancy and that persist for decades after birth. These "microchimeric" cells, named after the lion-goat-serpent hybrid of Greek mythology, have been found in every organ studied so far, though they are exceedingly rare: one such cell exists for every 10,000 to 1 million of a person's own cells. The cells were first noticed in the late 1800s when pathologist Georg Schmorl described placenta-like "giant cells" in the lungs of people who had died from eclampsia. In 1969, researchers detected Y-chromosome-containing white blood cells in people who would later give birth to boys. For more than two decades, scientists presumed these cells were temporary. That changed in 1993 when geneticist Diana Bianchi found Y-chromosome cells in women who had given birth to sons up to 27 years earlier. The cells appear to have regenerative properties, transforming into blood vessels or skin cells to promote wound healing. They also challenge a central assumption of immunology -- that the immune system classifies cells as either "self" or "non-self" and rejects foreign material. Microchimeric cells should trigger rejection but do not. Higher-than-typical concentrations have been found in people with autoimmune conditions including diabetes, lupus, and scleroderma.

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Mark Thomson, a professor of experimental particle physics at the University of Cambridge, takes over as CERN's director general this week, and one of his first major decisions during his five-year tenure will be shutting down the Large Hadron Collider for an extended upgrade. The shutdown starts in June to make way for the high-luminosity LHC -- a major overhaul involving powerful new superconducting magnets that will squeeze the collider's proton beams and increase their brightness. The upgrade will raise collisions tenfold and strengthen the detectors to better capture subtle signs of new physics. The machine won't restart until Thomson's term is nearly over. Thomson is far from disconsolate about the downtime. "The machine is running brilliantly and we're recording huge amounts of data," he told The Guardian. "There's going to be plenty to analyse over the period." Beyond the upgrade, Thomson must shepherd CERN's plans for the Future Circular Collider, a proposed 91km machine more than three times the size of the current collider. Member states vote on the project in 2028; the first phase carries an estimated price tag of 15 billion Swiss francs (nearly $19 billion).

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