Neuraminic acid, or sialic acid as it was first called, is a monosaccharide with nine carbons. It has a negative electric charge which gives compounds containing it a negative charge. This is useful for keeping molecules like red blood cells from getting to near to each other. The negative charge on the surface glycoproteins repels the red blood cell from each other or from the walls of blood vessels which also have compounds containing sialic acid.
Mature red blood cells have an active life for about seven days. White blood cells remove older red blood cells and de-sialylation of the surface proteins is one way the older cells are identified. Cancer cells with the ability to produce excess surface sialyation may have an increased chance to metastasize and turn up somewhere else in the body. [1, 3]
Our bodies need to be healthy and well enough nourished overall to keep the whole system working. The neuraminic acid is produced within our cells from other chemicals in a series of membranous channels called the endoplasmic reticulum and the golgi apparatus. The channels have embedded enzymes along the way somewhat like an assembly line in a factory. We can not just eat more sialic acid in our diet and have it show up on our cell surfaces – we have to be healthy enough and well enough nourished over all in order to be able to manufacture our own supply of sialic acid.
Therapeutic glycoproteins are being developed and the problem of just the right amount of sialylation is one of the hurdles being studied.  In addition to the negative charge sialic acid tends to stabilize and stiffen the protein portion of the glyco-compound. The proteins that line vessels were described to be somewhat like bottle-brushes; the protein being somewhat like the sturdy wire handle of the brush and with the negatively charged sialic acid acting as bristles that electrically repel other molecules of sialic acid. 
/This article was originally posted on 8/21/2013./ /Disclaimer: Information presented on this site is not intended as a substitute for medical care and should not be considered as a substitute for medical advice, diagnosis or treatment by your physician./
More recent research from the scientists at the University of Zurich, regarding sialic acid, found an association between the presence of autoimmune disease and reduced levels of sialic acid on the individual’s antibodies, which are important for the body’s immune cells to be able to recognize and remove infected or foreign or decaying cells: “Specific Sugar in Antibodies Structure Determines the Risk of Autoimmune Diseases,” Oct. 7, 2015, [molecularbiologynews.org]
- S.A. Brooks, M. V. Dwek, U. Schumacher, Functional and Molecular Glycobiology, (BIOS Scientific Publishers, Ltd., 2002), Amazon.
- Bork K, Horstkorte R, Weidemann W., “Increasing the sialylation of therapeutic glycoproteins: the potential of the sialic acid biosynthetic pathway.” J Pharm Sci. 2009 Oct;98(10):3499-508. doi: 10.1002/jps.21684. [ncbi.nlm.nih.gov]
- R. T. Almaraz, et. al., “Metabolic Flux Increases Glycoprotein Sialylation: Implications for Cell Adhesion and Cancer Metastasis.” Mol Cell Proteomics. 2012 July; 11(7): M112.017558. Published online 2012 March 28. doi: 10.1074/mcp.M112.017558 [ncbi.nlm.nih.gov]