Glycosylation is the process by which a
covalently attached to a target
lipids. This modification serves various functions.
 For instance, some proteins do not fold correctly unless they are glycosylated.
 In other cases, proteins are not stable unless they contain
oligosaccharides linked at the
nitrogen of certain
asparagine residues. The influence of glycosylation on the folding and stability of glycoprotein is twofold. Firstly, the highly soluble glycans may have a direct physicochemical stabilisation effect. Secondly, N-linked glycans mediate a critical quality control check point in glycoprotein folding in the endoplasmic reticulum.
 Glycosylation also plays a role in cell-to-cell adhesion (a mechanism employed by cells of the
immune system) via sugar-binding proteins called
lectins, which recognize specific carbohydrate moieties.
 Glycosylation is an important parameter in the optimization of many glycoprotein-based drugs such as
 Glycosylation also underpins the
ABO blood group system. It is the presence or absence of
glycosyltransferases which dictates which blood group
antigens are presented and hence what antibody specificities are exhibited. This immunological role may well have driven the diversification of glycan heterogeneity and creates a barrier to
zoonotic transmission of viruses.
 In addition, glycosylation is often used by viruses to shield the underlying viral protein from immune recognition. A significant example is the dense glycan shield of the envelope spike of the
human immunodeficiency virus.
Overall, glycosylation needs to be understood by the likely evolutionary selection pressures that have shaped it. In one model, diversification can be considered purely as a result of endogenous functionality (such as
cell trafficking). However, it is more likely that diversification is driven by evasion of pathogen infection mechanism (e.g.
Helicobacter attachment to terminal saccharide residues) and that diversity within the multicellular organism is then exploited endogenously.