A glycoprotein is a membrane-bounded protein to which are attached branching carbohydrates. Glycoproteins are:

 

Components of cell membranes, in which they extend throughout the lipid bilayer.
Important in cell-cell recognition, such as in human blood groups and immune system response.
Components of some body fluids, such as mucus, that are involved in lubrication.

 

Glycoproteins are formed in the Golgi apparatus in the process known as glycosylation.