Defensin: a multifunctional molecule lives up to its versatile name.

Human neutrophil proteins 1, 2 and 3 (HNP1-3) were originally identified as endogenous antibiotics that can kill microbial pathogens immediately after the onset of the host innate immune response. Recent studies revealed that these peptides perform additional, previously unexpected functions, notably the neutralization of certain secreted bacterial toxins. In this Opinion article, a brief overview of the well-established functions of HNP1-3 is given and novel biological activities of HNP1-3 are described, with emphasis on neutralization of secreted bacterial toxins. We propose that toxin neutralization represents a novel biological function of HNP1-3 in host defense.