The sugar group(s) can assist in protein folding or improve proteins' stability.
One example of glycoproteins found in the body are mucins, which are secreted in the mucus of the respiratory and digestive tracts. The sugars attached to mucins give them considerable water-holding capacity and also make them resistant to proteolysis by digestive enzymes.
Glycoproteins are important for white blood cell recognition, especially in mammals. Examples of glycoproteins in the immune system are:
molecules such as antibodies (immunoglobulins), which interact directly with antigens
molecules of the major histocompatibility complex (or MHC), which are expressed on the surface of cells and interact with T cells as part of the adaptive immune response.
Other examples of glycoproteins include:
components of the zona pellucida, which surrounds the oocyte, and is important for sperm-egg interaction.
structural glycoproteins, which occur in connective tissue. These help bind together the fibers, cells, and ground substance of connective tissue. They may also help components of the tissue bind to inorganic substances, such as calcium in bone.
Soluble glycoproteins often show a high viscosity, for example, in egg white and blood plasma.