Natural and induced SIgA: All observations of efficient bacterial clumping by IgA in vivo have used systems of vaccination or primary and secondary infections, with clump formation typically only observed in “immune” and not in “naive” animals.57,58 This suggests that “natural” IgA, using any of the definitions of “natural”, is not sufficient to drive bacterial cross-linking in vivo, at least for all species where this has been currently studied. This evidence concerns the gene CD79A and infection.