The majority of bacteria found within the gut are commensals, although it is unclear whether these organisms can elicit systemic immunity. New research indicates that gut-microbiota-specific serum antibodies targeting an epitope conserved among Gram-negative bacteria can protect the host from systemic pathogenic infection.
Soto, R., & Round, J. (2016, May 9). Immunology: You Remind Me of a Microbe i Know. Current Biology. Cell Press. https://doi.org/10.1016/j.cub.2016.03.064