Previous studies have shown that in the blood of healthy donors (1) there are no natural antibodies against sialylated glycoproteins, which contain Neu5Ac (N-acetylneuraminic acid) as the most widespread form of human sialic acid, and (2) there is a moderate level of antibodies capable of binding unnatural oligosaccharides, where Neu5Ac is beta-linked to a typical mammalian glycan core. In the present study, we investigated antibodies against Neu5Ac in more detail and verified the presence of Kdn (2-keto-3-deoxy-D-glycero-D-galacto-nonulosonic acid) as a possible cause behind their appearance in humans, taking into account the expected cross-reactivity to Kdn glycans, which are found in bacterial glycoconjugates in both the - and -forms. We observed the binding of peripheral blood immunoglobulins to sialyllactosamines (where sialyl is Kdn or neuraminic acid) in only a very limited number of donors, while the binding to monosaccharide Kdn occurred in all samples, regardless of the configuration of the glycosidic bond of the Kdn moiety. In some individuals, the binding level of some of the immunoglobulins was high. This means that bacterial Kdn glycoconjugates are very unlikely to induce antibodies to Neu5Ac glycans in humans. To determine the reason for the presence of these antibodies, we focused on noninfectious pathologies, as well as on a normal state in which a significant change in the immune system occurs: namely, pregnancy. As a result, we found that 2/3 of pregnant women have IgM in the blood against Neu5Ac2-3Gal1-4GlcNAc. Moreover, IgG class antibodies against Neu5Ac2-3Gal1-4GlcNAc and Neu5Ac2-6Gal1-4GlcNAc were also detected in eluates from the placenta. Presumably, these antibodies block fetal antigens.