This residue is negatively charged at physiological pH and is found in complex glycans on mucins and glycoproteins found at the cell membrane.

Along with involvement in preventing infections (mucus associated with mucous membranes—mouth, nose, GI, respiratory tract), Neu5Ac acts as a receptor for influenza viruses, allowing attachment to mucous cells via hemagglutinin (an early step in acquiring influenzavirus infection).

Neu5Ac is also important in the biology of a number of pathogenic and symbiotic bacteria[1][2][3] as it can be used either as a nutrient, providing both carbon and nitrogen to the bacteria, or in some pathogens, can be activated and placed on the cell surface.

[1] Bacteria have evolved transporters for Neu5Ac to enable them to capture it from their environment and a number of these have been characterized including the NanT protein from Escherichia coli,[4] the SiaPQM TRAP transporter from Haemophilus influenzae[5] and the SatABCD ABC transporter from Haemophilus ducreyi.

[6] In Japan, Neu5Ac is approved under the trade name Acenobel for the treatment of distal myopathy with rimmed vacuoles.