Immunity protein component of a toxin-immunity protein module, which functions as a cellular contact-dependent growth inhibition (CDI) system. Neutralizes the tRNase activity of cognate toxin WapA upon expression in E.coli. Does not inhibit WapA from other strains of B.subtilis. The WapA C-terminus cannot be expressed on its own in E.coli, however it can be cloned in the presence of its cognate immunity protein gene. Cell contact is necessary for growth inhibition (PubMed:23572593). Unlike the LXG toxin-immunity modules, WapAI mediates competition under shaking culture conditions (PubMed:34280190). Constitutively and highly expressed on solid and in liquid medium, with or without biofilm formation by 12 hours of culture. Repressed by DegU. A double wapA-wapI deletion strain is growth inhibited when cocultured with wild-type cells. When wapI is reintroduced it restores growth in a cognate toxin-dependent fashion (PubMed:23572593). Growth inhibition by wild-type cells is strongest on LB medium and less effective on media that promote biofilm formation (MSgg and 2xSGG) (PubMed:34280190).