We owe our lives to our immune systems – without it, even the most minor of sniffles could be fatal. But the immune system makes mistakes too, with the results ranging from allergies to multiple sclerosis. Now, researchers at the University of Toronto have found that cavities might also be collateral damage from an overzealous immune system.

Traditionally, bacteria have taken most of the blame for cavities and tooth decay. The bugs cling to your teeth as plaque and produce acid as waste, which dissolves tooth enamel, dentin and even filling material.

But the new study suggests the story is more complicated than that. Oral immune cells called neutrophils are dispatched by the body in response to invading bacteria – but the researchers found that they might be a little careless in the battle.

"It's like when you take a sledgehammer to hit a fly on the wall," says Yoav Finer, lead author of the study. "That's what happens when neutrophils fight invaders."

On their own, neutrophils can't damage teeth but the problems arise after acids from bacteria demineralize them. Once weakened, enzymes released by the neutrophils could wreak havoc on other tooth substances. Damage was found to appear in a matter of hours, and worse still, it also seems to apply to tooth-colored fillings, which may explain why they tend to fail within five to seven years.

"Ours is the first basic study to show that neutrophils can break down resin composites (tooth-colored fillings) and demineralize tooth dentin," says Russel Gitalis, first author of the study. "This suggests that neutrophils could contribute to tooth decay and recurrent caries."

The silver lining of the discovery is that it could lead to new types of treatment, or new standards for testing materials that are to be used in fillings.

The research was published in the journal Acta Biomateralia.

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