Sialic acid containing glycoproteins (sialoglycoproteins) bind selectin in humans and other organisms. Metastatic cancer cells often express a high density of sialic acid-rich glycoproteins. This overexpression of sialic acid on surfaces creates a negative charge on cell membranes. This creates repulsion between cells (cell opposition) and helps these late-stage cancer cells enter the blood stream. Recent experiments have demonstrated the presence of sialic acid in the cancer-secreted extracellular matrix.
Sialic acid-rich oligosaccharides on the glycoconjugates (glycolipids, glycoproteins, proteoglycans) found on surface membranes help keep water at the surface of cells.  The sialic acid-rich regions contribute to creating a negative charge on the cells' surfaces.  Since water is a polar molecule with partial positive charges on both hydrogen atoms, it is attracted to cell surfaces and membranes.  This also contributes to cellular fluid uptake. 
Sialic acid residues are present in the mucin glycoproteins of  mucus.
Sialic acid can "hide" mannose antigens on the surface of host cells or bacteria from mannose-binding lectin. This prevents activation of complement.
Sialic acid in the form of polysialic acid is an unusual posttranslational modification that occurs on the neural cell adhesion molecules (NCAMs).  In the synapse, the strong negative charge of the polysialic acid prevents NCAM cross-linking of cells.
Administration of estrogen to castrated mice leads to a dose-dependent reduction of the sialic acid content of the vagina. Conversely, the sialic acid content of mouse vagina is a measure of the potency of the estrogen. Reference substances are estradiol for subcutaneous application and ethinylestradiol for oral administration.