The third major constituent of the plasma membrane is always present on the cell's external surface, and it is attached either to lipids (making glycolipids) or proteins (making glycoproteins). About 2-60 monosaccharide units are composed of carbohydrate chains and can be branched or straight by structure. Carbohydrates with peripheral proteins create specialized sites on the surface of the cell that permits cells to identify one and all. These sites possess distinctive patterns which permit the cell to be identified, the same way facial features distinct to every individual let them be identified. This identification is essential for cells as it lets the immune system distinguish between foreign tissues or cells (known as non-self) and the body's cell (Known as self). The same types of glycolipids and glycoproteins are present on the superficial of viruses that change consistently, protecting immune cells from recognition and invading them.