Red Cell Membrane and Its Disorders

A comprehensive vocabulary review deck covering erythrocyte membrane architecture, cytoskeletal proteins, lipid bilayer dynamics, and associated inherited and acquired red cell membranopathies.

Biconcave Red Blood Cell Dimensions

The structural dimensions of a mature normal erythrocyte, featuring a diameter of approximately $7.2\text{--}7.5,\mu m$, a thickness of $2.0\,\mu m$ at the peripheral rim and $1.0\,\mu m$ at the center, a total surface area of approximately $136\,\mu m^2$, and an average cell volume of $90\,fl$.

Glycocalix

A carbohydrate-rich exterior coat covering the erythrocyte surface composed of seven distinct monosaccharides (N-acetylneuraminic acid, fucose, glucose, galactose, mannose, N-acetylglucosamine, and N-acetylgalactosamine) linked to proteins and lipids, imparting a net negative surface charge.

Lipid Bilayer Asymmetry

The unequal transbilayer distribution of phospholipids across the erythrocyte membrane, featuring phosphatidylcholine and sphingomyelin predominantly in the outer monolayer, and phosphatidylethanolamine and phosphatidylserine along with minor phosphoinositides restricted to the inner monolayer, maintained by ATP and reduced glutathione.

Phosphatidylserine (PS) Exposure

The loss of normal membrane phospholipid asymmetry that presents phosphatidylserine on the external monolayer, serving as a recognition marker for macrophages to phagocytose senescent or damaged erythrocytes.

Vertical Interactions

Perpendicular structural linkages that anchor the underlying membrane skeleton to the overlying lipid bilayer via integral transmembrane proteins, primarily mediated by Band 3-ankyrin-protein 4.2 complexes and glycophorin C-protein 4.1R complexes.

Horizontal Interactions

Planar structural connections within the two-dimensional submembrane cytoskeleton itself, including the self-association of spectrin heterodimers into tetramers and the junctional binding of spectrin to short actin filaments and protein 4.1.

Membrane Cytoskeleton Network

A hexagonal scaffolding beneath the lipid bilayer consisting of long spectrin tetramers linked at junctional complexes with short actin protofilaments and stabilized by linker proteins such as ankyrin and adducin, conferring mechanical elasticity and shape stability.

RBC Membrane Proteins and Associated Hemolytic Anemias

A systematic classification of structural proteins and transport channels (such as $\alpha$-spectrin, $\beta$-spectrin, ankyrin, Band 3, and protein 4.1) along with their corresponding gene loci, whose genetic deficiencies or alterations lead to membranopathies including hereditary spherocytosis, hereditary elliptocytosis, and hereditary pyropoikilocytosis.

Spectrin

The principal cytoskeletal protein of the red cell consisting of $\alpha$ and $\beta$ polypeptide chains organized as triple-helical repeats ($20$ in $\alpha$-spectrin, $16$ in $\beta$-spectrin); $\alpha\beta$ dimers associate head-to-head to form tetramers that tether to actin filaments.

Band 3 (Solute Carrier Family 4 Member 1)

A major multi-pass transmembrane glycoprotein that functions as an anion exchange channel for bicarbonate and chloride, displays blood group activity, binds glycolytic enzymes, and mediates vertical attachment to ankyrin and protein 4.2.

Ankyrin (Band 2.1)

A cytoskeletal linker protein encoded by the $ANK1$ gene on chromosome $8p11.2$ that connects the $\beta$-chain of spectrin to the cytoplasmic domain of transmembrane Band 3, providing crucial vertical stability.

Protein 4.1R

A peripheral cytoskeletal protein encoded by $EPB41$ on chromosome $1p36.2\text{--}p34$ that stabilizes the horizontal interaction between spectrin and actin, and connects the skeleton vertically to glycophorin C.

Membrane Viscoelasticity

The physical capability of the red blood cell membrane to behave both as an elastic solid that recoils back to its biconcave shape via the spectrin-actin cytoskeleton and as a viscous liquid that exhibits time-dependent deformation and creep under sheer stress.

Cytoplasmic Viscosity

The internal flow resistance of an erythrocyte, determined predominantly by intracellular hemoglobin concentration and mean corpuscular hemoglobin concentration (MCHC); marked elevations impair deformability and increase cell rigidity.

Hereditary Spherocytosis (HS)

An inherited hemolytic anemia, predominantly autosomal dominant, caused by defective vertical protein interactions (such as mutations in ankyrin, Band 3, spectrin, or protein 4.2) leading to lipid loss, reduced surface area-to-volume ratio, decreased deformability, and splenic sequestration.

Spherocytes (Peripheral Blood Film)

Hyperdense, spherical red blood cells on peripheral smear that exhibit a reduced diameter and completely lack central pallor due to membrane surface area loss relative to intracellular volume.

Osmotic Fragility Test in Hereditary Spherocytosis

A diagnostic in vitro assay measuring erythrocyte lysis across decreasing concentrations of sodium chloride ($NaCl$), demonstrating an upward and rightward curve shift due to premature bursting of spherocytes with reduced surface area-to-volume reserve.

Acidified Glycerol Lysis Test (AGLT)

A laboratory screening assay for hereditary spherocytosis that utilizes glycerol to slow the rate of water entry into erythrocytes in vitro, measuring the delayed time required for osmotic lysis.

Hereditary Elliptocytosis (HE)

An autosomal dominant erythrocyte membranopathy caused by horizontal cytoskeletal defects, most frequently structural alterations in domain 1 of $\alpha$-spectrin that prevent spectrin dimer-dimer self-association, manifesting with prominent elliptical red cells on peripheral smear.

Hereditary Pyropoikilocytosis (HPP)

A severe form of hereditary elliptocytosis characterized by a marked deficiency of spectrin combined with defective spectrin self-association, abnormal red cell thermal sensitivity at elevated temperatures, and prominent poikilocytes and fragments on blood film.

Stomatocytes

Uniconcave, bowl-shaped red cells that exhibit a slit-like, mouth-shaped central pallor on dried films, produced by preferential expansion of the inner half of the membrane lipid bilayer and accompanied by excessive cation leakage and risk of pseudohyperkalemia.

Acanthocytes

Densely contracted red blood cells that lack central pallor and possess several irregularly spaced, thorny spicules of varying lengths and widths protruding from the cell surface.

Spur Cell Anaemia

A severe hemolytic state occurring in end-stage liver disease caused by passive acquisition of excess membrane cholesterol, which increases the cholesterol-to-phospholipid ratio, reduces red cell deformability, and causes splenic destruction of thorny acanthocytes.

Abetalipoproteinemia

A rare autosomal recessive disorder characterized by acanthocytosis on peripheral smear, defective intestinal fat absorption, and severe systemic hypolipidemia.

McLeod Phenotype

An X-linked condition resulting from mutations in the $XK$ gene, which encodes a structural membrane protein required for normal Kell antigen expression, presenting with $8\%\text{ to }85\%$ acanthocytosis and mild compensated hemolysis.

Target Cells (Codocytes)

Discoid red cells displaying a central area of hemoglobin within the zone of central pallor resembling a bull's eye, resulting from an increased surface area-to-volume ratio due to either excess membrane lipid accumulation or reduced cytoplasmic volume.

Echinocytes (Burr Cells)

Erythrocytes with numerous, fine, uniform, and regularly spaced projections across the entire cell periphery, caused by expansion of the outer leaflet of the lipid bilayer relative to the inner leaflet, or produced in vitro as an artifact of contact with glass slides.

Complement Control Proteins (DAF and MIRL)

Cell surface protective glycoproteins consisting of CD55 (Decay Accelerating Factor), which inhibits C3 convertases, and CD59 (Membrane Inhibitor of Reactive Lysis), which prevents assembly of the membrane attack complex (MAC).