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Cells cast as villains in severe allergic reactions may actually play an important role in protecting the body from infection, doctors said Wednesday.

Mast cells are known to be involved in the inflammation that accompanies allergic reactions. Severe allergic reactions in response to anything from a bee sting to peanuts, known as anaphylactic shock, can kill.Scientists have long tried to find out why the body is capable of such a strong and deadly response. The cells involved must have a very important function, they reasoned - but no one knew what else mast cells did.

Lothar Hultner and colleagues at GSF Institut fur Experimentelle Hamatologie in Munich said they thought they may have found the answer.

They bred mice to produce fewer mast cells, then gave them an infection. Those with fewer mast cell all died after five days, while only one-fourth of their normal litter mates did.

Mast cells store tumor necrosis factor (TNF), a hormone that has many roles in the immune system, in a form that is released very quickly.

TNF also has a role in allergic reactions and in diseases such as rheumatoid arthritis in which the immune system seems to attack the body by mistake.

"We consider that this unique property of mast cells may be critically important here," Hultner's group wrote in the science journal Nature.

They said it was also possible that mast cells might have a role in the deployment of immune system cells that destroy invading bacteria.

Soman Abraham and colleagues at the Washington University School of Medicine in St. Louis reported similar findings.

Stephen Galli and Barry Wershil of Beth Israel Hospital in Boston said the findings shed light on a mysterious subject.

"The two new papers demonstrate that mast cells represent a central component of host defense against bacterial infection," they said in a commentary in Nature.