Immunity protein component of a toxin-immunity protein module, which functions as a cellular contact-dependent growth inhibition (CDI) system. CDI modules allow bacteria to communicate with and inhibit the growth of closely related neighboring bacteria in a contact-dependent fashion. Bacteria that have this module inhibit or kill bacteria without it, giving them a growth advantage. Specifically inhibits the toxic activity of cognate toxin DddA (C-terminal 163 residue fragment) upon expression in E.coli. The toxic domain forms a 1:1 complex with the DddI immunity protein. This protein blocks the active site of the toxin. A double dddA-dddI deletion has a 100-fold growth disadvantage compared to wild-type in competition experiments.