Immune cells carry concealed weapons. (Biology).

For decades, immunologists thought they knew how bacteria-killing cells called neutrophils finished off the microbes they engulfed: Through a so-called respiratory burst, the cells attack bacteria inside them with highly reactive oxygen molecules known as free radicals and with hypochlorous acid, the agent in common laundry bleach. A new study challenges that scenario, however.

In the March 21 Nature, Anthony W. Segal of University College London and his colleagues propose that protein-cleaving enzymes called proteases are the real microbe destroyers in neutrophils. The scientists present evidence that the respiratory burst triggers a surge of potassium ions rushing into the cell compartment where engulfed microbes are held. This ion flow, in turn, releases proteases that had been bound to other molecules and thus were inactive.

In an accompanying commentary, Walter Gratzer of King's College London says that the findings "cast new and unexpected light on one of the cornerstones of the immune system"--J.T.