In today’s PLoS One, Robert Moir and others at Massachusetts General Hospital in Charlestown formally publish a finding they had earlier presented at the 9th International Conference AD/PD held last spring in Prague. Far from being merely a catabolic byproduct, these scientists assert, the amyloidogenic peptide Aβ serves the physiological function of fighting off microbes as part of the body’s innate immune system. First author Stephanie Soscia and colleagues report experiments comparing Aβ’s activity against a range of clinically important microorganisms. For a detailed story of the context, clues from the microbiology literature, and this in vitro data, see ARF conference news. In the paper, the authors add new results from experiments comparing antimicrobial activity in high-Aβ temporal lobe samples and low-Aβ cerebellar samples from people who had had Alzheimer disease versus age-matched controls.—Gabrielle Strobel.