Cellular receptors for IgA (FcαR) mediate important protective functions. An extensive panel of site-directed mutant IgAs was used to identify IgA residues critical for FcαR (CD89) binding and triggering. Although a tail-piece-deleted IgA1 was able to bind and trigger CD89, antibodies featuring CH3 domain exchanges between human IgA1 and IgG1 could not, indicating that both domains but not the tailpiece are required for FcαR recognition. To further investigate the role of the interdomain region, numerous IgA1s, each with a point substitution in either of two interdomain loops (Leu-257-Gly-259 in Cα2; Pro-440-Phe-443 in Cα3), were generated. With only one exception (G259R), substitutions produced either ablation (L257R, P440A, A442R, F443R) or marked reduction (P440R) in CD89 binding and triggering. Further support for involvement of these interdomain loops was provided by interspecies comparisons of IgA. Thus a human IgA1 mutant, LA441- 442MN, which mimicked the mouse IgA loop sequence through substitution of two adjacent residues in the Cα3 loop, was found, like mouse IgA, not to bind CD89. In contrast, bovine IgA1, identical to human IgA1 within these interdomain loops despite numerous differences elsewhere in the Fc region, did bind CD89. We have thus identified motifs in the interdomain region of IgA Fc critical for FcαR binding and triggering, significantly enhancing present understanding of the molecular basis of the IgA-FcαR interaction.