Antibodies are one of the body’s most effective defenses against microbial pathogens. But at least one of these immune system proteins helps an apparently harmless bacterium make itself a lasting home in the human gut. The bacterium seems to coax immunoglobulin A (IgA), one of the most abundant antibodies produced by mammals, to cover its surface, helping it stick to the mucus lining of the gut and become a stable part of the microbiome, the constellation of microbes inhabiting our gut. The finding, in germ-free mice, could one day help researchers trying to treat a variety of conditions by adding microbes to the human body.
This “dialogue” between the bacteria and the immune system “will likely influence the function of many physiological systems,” says Sidonia Fagarasan, an immunologist at the RIKEN Yokohama Institute in Japan, who first proposed in 2002 that IgA might help certain bacteria rather than eliminate them.
IgA was discovered 50 years ago. A human makes 3 to 5 grams of this antibody daily, some 75% of the body’s antibody production. IgA is also abundant in mother’s milk, presumably to fend off infections. But after noticing that mice with low levels of IgA had abnormal microbiomes, Fagarasan proposed that IgA also
played a role in maintaining and controlling bacteria in the body. But no one had been able to pin down how, until now.
