<P> Red blood cells are deformable, flexible, are able to adhere to other cells, and are able to interface with immune cells . Their membrane plays many roles in this . These functions are highly dependent on the membrane composition . The red blood cell membrane is composed of 3 layers: the glycocalyx on the exterior, which is rich in carbohydrates; the lipid bilayer which contains many transmembrane proteins, besides its lipidic main constituents; and the membrane skeleton, a structural network of proteins located on the inner surface of the lipid bilayer . Half of the membrane mass in human and most mammalian red blood cells are proteins . The other half are lipids, namely phospholipids and cholesterol . </P> <P> The red blood cell cell membrane comprises a typical lipid bilayer, similar to what can be found in virtually all human cells . Simply put, this lipid bilayer is composed of cholesterol and phospholipids in equal proportions by weight . The lipid composition is important as it defines many physical properties such as membrane permeability and fluidity . Additionally, the activity of many membrane proteins is regulated by interactions with lipids in the bilayer . </P> <P> Unlike cholesterol, which is evenly distributed between the inner and outer leaflets, the 5 major phospholipids are asymmetrically disposed, as shown below: </P> <Ul> <Li> Phosphatidylcholine (PC); </Li> <Li> Sphingomyelin (SM). </Li> </Ul>

The mature red blood cell contains a nucleus