Final answer:
A glycoprotein is the structure on the cell membrane consisting of sugar and protein that helps with immune system cell recognition by forming the specialized sites on the surface of the cell.
Step-by-step explanation:
The structure on the outside of the cell membrane that is made of part sugar and part protein, and aids in cell recognition by the immune system, is known as a glycoprotein. Glycoproteins are integral to the plasma membrane and have chains of carbohydrates that project outside the cell, which help cells to recognize each other. These carbohydrate chains vary in length from 2-60 monosaccharide units and can be either straight or branched. Together with peripheral proteins, glycoproteins form specialized sites on the cell surface that are crucial for the immune system to differentiate between self and non-self entities.
This recognition is essential for protection against pathogens. Additionally, glycolipids are similar to glycoproteins but are attached to lipids instead of proteins. Both glycoproteins and glycolipids are part of the glycocalyx, which acts as a 'sugar coating' and facilitates cell-to-cell communication, adhesion, and interaction with the watery environment.
The structure responsible for cell recognition by the immune system is a glycoprotein, which is found on the cell membrane and composed of sugar chains attached to proteins.