Important differences between humans and other primates were found to be their particular cell surface molecules. Most cell surfaces have a variety of molecules, such as various sugars, projecting from their surface serving a multitude of functions. In a sense the cellular surface is like a dynamic forest with carbohydrates being many structures protruding from the cell. This organization and the dynamic nature of these surface molecules is key for molecule binding. An example of this is how colony–stimulating factors (CSFs) bind to surface molecules of stem cells in order to trigger cell division.

Watch the animated video "Close-up of the Surface of a Stem Cell"

One of the carbohydrate types that can be found on the cell surface is sialic acid. Sialic acids are nine-carbon sugars that can have acetylated, sulfated, methylated, and lactylated derivatives. The modification in structure is unmatched amongst the sugars. There are many combinations of different glycosidic bonds that result in hundreds of ways in which sialic acids can appear on glycoconjugates. For instance, sialic acids are often found as the terminal carbohydrate residue on glycoproteins.

Chemical structure of N-Acetylneuraminic acid (sialic acid).
Image credit: Nature Chemical Biology from nature.com

In the 1950s it was discovered that sialic acid on the surface of host cells functions as receptors for the influenza virus. During the process of infection the virus binds to the cell surface, the viral haemagglutinin protein digests sialic acid, and then the virus can enter the cell. After viral replication within the cell the viral neuraminidase protein removes the sialic acid that, in turn, allows the progeny virions to easily exit the cell. Since the virus needs to clip off the sialic acid to leave the infected cell, if the action is blocked then the virus is stopped. This clipping event can be used as a target for drug development.

Structure of an influenza virus.
Image credit: http://www.proteopedia.org/wiki/index.php/Influenza#cite_note-3

In addition to humans, particular strains of the influenza virus can infect a variety of both domestic and wild animals including birds, pigs, and horses. With bird influenza, for example, the interaction of the virus with sialic acid is only subtly different than the interaction of human-adapted influenza virus with sialic acid. Strains of influenza virus adapted to birds binds to sialic acid receptors preferentially with α (2–3) linked sugars compared to human-adapted influenza viruses which are thought to bind receptors with α (2–6) linkages. The difference from avian receptor configuration requires only 1 amino acid change of the virus, and all 5 of the sequenced 1918 viruses have this change.

The influenza virus enters the cell by binding to sialic acid.
Image credit: http://speedy.st-and.ac.uk/~naismith/teaching/lectures/ch5614/drugs_design.htm

There is a promising future in glycobiology research and application. In addition to the role of glycans in pathogen-host interactions, they also provide evidence of rates of molecular change over geological time. Carbohydrates, by their unique branching structure, contain an evolutionary potential of information much greater than other types of biological oligomers. When added to other information sources including protein and nucleic acid molecular analysis, morphological characteristics, and fossil records, the addition of glycan molecular analysis can contribute to a more complete understanding of primate evolution.

A colored transmission electron micrograph of Streptococcus bacterium attached to a human tonsil cell. As with the influenza virus, sialic acid is necessary for this pathogen's binding action.
Image credit: http://www.biologyreference.com/Ar-Bi/Bacterial-Cell.html