The red cell membrane and chemistry of blood group antigens

The red cell membrane is composed of about 40% (w/w) lipids and up to 10% carbohydrates, the remainder being proteins. The exact arrangement of its components is still unresolved, but a rough model is shown in Figure 14.1.

The lipids of the red cell membrane can be subdivided into 60% (w/w) phospholipids, 30% (w/w) neutral lipids (mainly cholesterol) and 10% (w/w) glycolipids. The phospholipids and glycolipids play a role in the structure of the membrane and are thought to be important in the maintenance of red cell shape. These lipids have a molecular arrangement reminiscent of a tuning fork, with the hydrophobic fatty acids forming the 'prongs' and the polar group the 'handle'. They are arranged in a bilayer with the 'prongs' pointing inwards and the hydrophilic 'handle' pointing out towards the plasma or towards the cytoplasmic surface of the membrane. Cholesterol is inserted between the other lipids. This arrangement allows the interior of the membrane to be in a semifluid state and the whole membrane to be very flexible. The lipid bilayer does not allow the passive transfer of ions.

The carbohydrates are attached to the lipids and proteins, and occur only on the external surface of the membrane. They are

Figure 14.1 Diagrammatic representation of red cell membrane.

Membrane Blood Group Antigens

composed of chains of monosaccharides, the majority of which are hexoses.

About 20-40% of the proteins of the membrane are released relatively easily (e.g. by changing the ionic strength of the medium) and are therefore not very firmly attached (peripheral proteins). The remaining 60-80% are released only after drastic treatment with detergents or bile salts; these integral proteins penetrate the lipid bilayer and, in some cases, are bound to the cytoskeleton. After red cell membranes have been treated with the detergent sodium dodecyl sulphate, the proteins can be separated electrophoretically on a polyacrylamide gel according to their molecular mass. If the gel is stained for protein, up to eight bands are seen, as shown in Figure 14.2. Bands 1 and 2 are proteins that are easily released by a low-ionic-strength medium. They are monomers and dimers of the contractile protein spectrin, which form a network of tetramers on the inner surface of the membrane, contributing to the maintenance of the red cell shape. Band 5 (actin) links the spectrin tetramers together. Band 3, the anion exchanger, exists in the membrane as dimers and higher oligomers linked to the cytoskeleton through ankyrin (band 2.1), band 4.2 and band 4.1.

If the polyacrylamide gel is stained for sialic acid with periodic acid-Schiff (PAS) stain, four or five bands are seen (Figure 14.2). These bands contain four sialic acid-rich glyco-proteins called glycophorins, and their dimers. The genetically related glycophorins (GP), GPA and GPB, exist as monomers, dimers and as a heterodimer of GPA and GPB. Additional bands represent the minor glycophorins, GPC and its truncated isoform, GPD. The genes for all these glycophorins have been cloned and sequenced, and the amino acid sequence deduced. All of the carbohydrate of these molecules (60%, w/w) is located on the external domains, which extend far beyond the lipid bilayer, allowing charged groups to extend some distance into

Glycophorin A2

Glycophorin AB Glycophorin B2 Glycophorin A Glycophorin C

Glycophorin D Glycophorin B

CB PAS

SDS gels

Figure 14.2 The proteins of the red cell membrane after separation by electrophoresis in sodium dodecyl sulphate (SDS) gels. (a) Stained for protein (Coomassie blue, CB); (b) stained for carbohydrate (periodic acid-Schiff reagent, PAS).

the plasma. The molecules cross the lipid bilayer once. GPA, GPC and GPD extend well into the cytosol and appear to interact with the cytoskeleton, whereas GPB extends only a short distance into the cytosol (Figure 14.1).

Blood group antigens have been found in the polypeptide and carbohydrate moieties of membrane glycoproteins and in the carbohydrate moieties of glycolipids (Table 14.1). Most blood groups represent amino acid sequence changes in glycoproteins,

Spectrin

Band 3 (anion channel) 4.1 4.2

Table 14.1 Blood group active proteins.

Protein

Blood group

Mol. wt (kDa)*

Copies per cell (x103)

Function

Band 3 (CD233)

Diego; ABHf

100

1000

Anion transport; cytoskeletal attachment

UT-B1

Kidd, ABHf

46-60

14

Urea transport

Aquaporin 1

Colton, ABHf

40-60

200

Water channel

Aquaporin 3

GIL

45

Glycerol and water channel

GLUT-1

ABHf

55

500

Glucose transport

RhD and RhCcEe proteins

Rh

30-34

100-200

Involved in NH + or CO2 transport?

(CD240D and CE)

RhAG (CD241)

Duclos, ABHf

35-100

100-200

Involved in NH + or CO2 transport?

Kx protein

Kx

37

Neurotransmitter transport?

Duffy gp (CD234)

Duffy

40-50

6-12

Chemokine receptor

Lutheran gp (CD239)

Lutheran

78 and 85

2-4

Adhesion (to laminin)

ICAM-4

LW

37-47

3-5

Adhesion

CD44

Indian

80

6-10

Adhesion (to hyaluronan)

Xg and CD99 gps

Xg

23-28 and 32

1-10

Adhesion?

ERMAP

Scianna

60-68

Adhesion?

EMMPRIN (CD147)

Ok

35-68

Signal transduction?

CDw108

JMH

76

1-3

Adhesion?

Kell gp (CD238)

Kell

93

4-8

Endopeptidase

Acetylcholinesterase

Yt

72

7-10

Acetylcholinesterase

Dombrock gp

Dombrock

47-57

ADP-ribosyltransferase

DAF (CD55)

Cromer

70

6-14

Complement regulation

CR1 (CD35)

Knops

190

0.2-1

Complement regulation

Glycophorin A (CD235A)

MN

43

1000

Sialic acid carrier

Glycophorin B (CD235B)

Ss

25

250

Sialic acid carrier

Glycophorins C and D (CD236)

Gerbich

40 and 30

143 and 82

Sialic acid carrier; cytoskeletal attachment

"Apparent molecular weight determined by electrophoresis. Carbohydrate antigens. gp, glycoprotein.

"Apparent molecular weight determined by electrophoresis. Carbohydrate antigens. gp, glycoprotein.

although the Rh antigens are non-glycosylated proteins containing two or three molecules of palmitic acid. ABH and Ii antigens are found predominantly on the carbohydrate moieties of the major red cell glycoproteins band 3 (anion exchanger) and on GLUT-1, the glucose transporter, although they are also present on some other minor glycoproteins and on the carbohydrate portions of membrane glycolipids. P, P1 and PK antigens are expressed on the carbohydrate of glycolipids. The M and N antigens arise from interactions between the carbohydrate and polypeptide in the glycoprotein GPA. In addition to the antigens on integral membrane components, some are adsorbed passively from plasma (e.g. Lewis, Chido/Rogers). Structural details of selected antigens are given in Chapter 15.

Originally, an antigen was defined as the part of a molecule that is bound by a specific antibody. More recently, it has become customary to define an antigen as a substance that can stimulate an immune response (immunogenicity). Immune responses can be either positive or negative. Positive responses lead to the production of antibodies (humoral immunity) and/or proliferation of immunocompetent cells (cellular immunity) that can bind and eliminate their stimulatory antigen. In negative responses, the cells that mediate humoral and cellular immune responses are rendered non-responsive. This state is described as acquired immunological tolerance and is important in preventing autoimmune disease, as well as in establishing the 'take' or acceptance of transplanted syngeneic and allogeneic tissues.

The hallmark of the adaptive immune response is its specificity: specific immunocompetent cells and/or antibodies are produced, which can distinguish molecules that differ only by two or three atoms (e.g. TNP versus DNP). As described later, even the difference between the A and B antigens is minimal. However, antibodies can often react with antigens similar to, but not identical to, the stimulatory antigen (cross-reactivity). As far as is known, specific immune responses to human red cells are

How To Bolster Your Immune System

How To Bolster Your Immune System

All Natural Immune Boosters Proven To Fight Infection, Disease And More. Discover A Natural, Safe Effective Way To Boost Your Immune System Using Ingredients From Your Kitchen Cupboard. The only common sense, no holds barred guide to hit the market today no gimmicks, no pills, just old fashioned common sense remedies to cure colds, influenza, viral infections and more.

Get My Free Audio Book


Responses

  • Annalise
    What antigens are seen on red cell membrane?
    3 years ago
  • futsum
    What is the arrangement of the A antigen on the red cell?
    3 years ago

Post a comment