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. Protects cells against the CDI activity of CdiA, its cognate toxin protein, but not against non-cognate CdiA from E.coli strains 563, EC93 or D.dadantii strain 3937. Probably interacts with cognate toxin CdiA.