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. Neutralizes the toxic activity of cognate toxin CdiA-NC101 (the C-terminal 154 residue CT fragment) (PubMed:26305955). Does not inhibit toxic activity of CdiA from other toxin-immunity modules or strains of E.coli (Probable). Mediates dimerization of the ternary CdiA-CT-NC101, CdiI-NC101 and EF-Tu complex; both CdiI molecules contact both EF-Tu molecules (PubMed:28973472). Forms a contact-dependent growth inhibition complex of CdiA-CT-NC101, CdiI-NC101 and EF-Tu; the complex is a dimer of heterotrimers.