A carbohydrate linked covalently to a protein. Formed in the Golgi apparatus in the process of glycosylation, glycoproteins are important components of plasma membranes, in which they extend throughout the lipid bilayer. They are also constituents of body fluids, such as mucus, that are involved in lubrication. Many of the hormone receptors on the surfaces of cells are glycoproteins. Glycoproteins produced by viruses attach themselves to the surface of the host cell, where they act as markers for the receptors of leucocytes. Viral glycoproteins can also act as target molecules and help viruses to detect certain types of host cell; for example, a glycoprotein on the surface of HIV enables the virus to find and infect white blood cells.