Abstract: Defensins are antimicrobial peptides that contribute broadly to innate immunity, including protection of mucosal tissues. &nbs ;Human &nbs ; However, in contrast to HD5 and all other defensins, HD6 lacks bactericidal activity. &nbs ;Nevertheless, we found that HD6 affords protection against invasion by enteric bacterial pathogens in vitro and in vivo. &nbs ;After stochastic binding to bacterial surface proteins, HD6 undergoes ordered self-assembly to form fibrils and nanonets that surround and entangle bacteria. &nbs ;This self-assembly mechanism occurs in vivo, requires histidine-27, and is consistent with X-ray crystallography data. &nbs ;The composite data support a key role for HD6 in protecting the small intestine against invasion by diverse enteric pathogens, and may explain the conservation of HD6 throughout Hominidae evolution.