Red cell antigens:
The surface of the red cell consists of a bilipid membrane in which large
protein molecules are embedded. The shape of the cell, along with its
deformability, is maintained by skeletal proteins lying on the internal side of
the bilipid layer. Haemoglobin fills the space surrounded by this complex
membrane. The biconcave dist shape of the red blood cell maximizes the
cell surface area.
The bilipid membrane is composed of a class of molecule called phospholipids.
Phospholipids are amphipathic; that is, they have properties which are
both hydrophilic and hydrophobic. The polar heads of the molecule are
hydrophilic and the hydrocarbon tails are hydrophobic. It is natural for
phospholipids to form sheets or bilipid layers with the hydrophobic tails of
each molecule sequestered with in the bilipid layer. The hydrophilic heads
are on the two surfaces of the bilipid layer-one surface exposed to the
surrounding plasma while the other is exposed to hemoglobin. The membrane
is symmetric in that certain phospholipids dominate the outer layer and others
are predominantly on the inner layer. The membrane is described as fluid
because the lipid molecules actively migrate within the membrane parallel to the
surface. they very rarely "flip flop" from the outside to the
inside or vice versa. This fluidity can be illustrated by showing that
when two cells are fused their membranes completely integrate within about one
hour.
Most of the protein molecules in the bilipid membrane have oligosaccharides
attached in very precise positions. Oligosaccharides are groups of sugar
molecules. Some of these are known to be blood group antigens. Others
serve as sites for metabolic functions of the red cell but have no recognized
blood group specificity.
The most important blod group antigens are A, B and D (Rho). A and B
antigens are well defined chemically. The antigenic determinants are the
terminal sugars of oligosaccharide chains attached either directly to the cell
membrane or attached to protein chains which protrude from the bilipid layer.
XWhen the oligosaccharide chains are attached to the cell membrane, the
structure is called a glycolipid molecule, and when oligossacharides are
attached to a protein structure, the molecule is called a glycoprotein, The
Rh antigens are much well less defined but they are primarily protein and appear
to be embedded in the bilipid membrane with the portions of the molecule exposed
on the outer surface of the cell. They are probably associated with one of
the protein chains in the membrane which has already been partially
characterized.
The structure of all erythrocyte antigens in under genetic control. If a
particular gene is inherited, the corresponding antigen is assembled and
inserted into the membrane. Often a group of genes works in concert to
produce a particular red blood cell antigen. The process of antigen of antigen
production occurs during red blood cell maturation in the bone marrow prior to
the loss of the red cell nucleus.
