Certain immune cells backstab their cellular coworkers during allergic reactions.

Mast cells, the security patrols of the immune system, can trigger allergic inflammation when they run into unfamiliar proteins (SN: 9/5/07). For the first time, researchers have seen these overeager cells attract and trap other key immune cells, then expel their hostages’ inflammatory chemicals in a process dubbed nexocytosis. The discovery may help scientists understand the root causes of health conditions involving allergies or the immune system, such as asthma and dermatitis.

When mast cells sense proteins from outside the body, they instigate an inflammatory response by expelling granules, packages of chemicals that attract other immune cells — including neutrophils, a key type of white blood cell (SN: 9/23/24).

Curious about how mast cells and neutrophils interact, immunologist and biochemist Michael Mihlan and colleagues triggered mast cells to react to a human blood protein. The team then watched the reactions in both mouse and human cells through a microscope. Mast cells lured neutrophils toward them, then swallowed the specialized white blood cells, the researchers reported in the Sept. 19 Cell. “This was a total surprise to us,” says Mihlan, of the University of Münster in Germany.

Even after trapped neutrophils died, their proteins, DNA and other molecules remained inside their mast cell captors. The mast cells could then use these leftovers to sustain themselves. Or, they could perform nexocytosis by spewing out inflammatory neutrophil chemicals alongside granules when another allergic reaction began.

In the future, Mihlan hopes to learn whether mast cells’ trapping strategy is a force for good in some cases, such as fighting off foreign bacteria.