FREE ESSAY ON SICKLE CELL ANEMIA

SICKLE CELL ANEMIA

By Gerry VanCleve and Kevin Lemerond
Pages with Brief Description
Page 1:Cover 
Page 2Pages with brief Description
Page 3:A Brief History of Sickle Cell Disease
Page4-5: How Does Sickle Cell Cause Disease?
Page 6: The Sickle Red Cell
Page 7:How Do People Get Sickle Cell Disease?
Page 8-9:Do Factors Other than Genes Influence Sickle Cell Disease?
Page 10:Overview
Page 11-15: Hemoglobin F Levels and Amelioration of Sickle Cell Disease
Page 16-17:References
The reason why this report is not double-spaced is because of the lengthy effect it would
have upon the reader. We feel that this report looks a lot better single-spaced.
A Brief History of Sickle Cell Disease
Sickle Cell Disease in African Tradition
Sickle cell disease has been known to the peoples of Africa for hundreds, and perhaps
thousands, of years. In West Africa various ethnic groups gave the condition different
names:
Ga tribe:
Chwechweechwe
Faute tribe:
Nwiiwii
Ewe tribe:
Nuidudui
Twi tribe:
Ahotutuo
Sickle Cell Disease in the Western Literature
Description of Sickle Cell Disease 
In the western literature, the first description of sickle cell disease was by a Chicago
physician, James B. Herrick, who noted in 1910 that a patient of his from the West Indies
had an anemia characterized by unusual red cells that were sickle shaped. 
Relationship of Red Cell Sickling to Oxygen 
In 1927, Hahn and Gillespie showed that sickling of the red cells was related to low
oxygen. 
Deoxygenation and Hemoglobin 
In 1940, Sherman (a medical student at Johns Hopkins) noted a birefringence of
deoxygenated red cells, suggesting that low oxygen altered the structure of the
hemoglobin in the molecule. 
Protective Role of Fetal Hemoglobin in Sickle Cell Disease 
Janet Watson, a pediatric hematolist in New York, suggested in 1948 that the paucity of
sickle cells in the peripheral blood of newborns was due to the presence of fetal
hemoglobin in the red cells, which consequently did not have the abnormal sickle
hemoglobin seen in adults. 
Abnormal Hemoglobin in Sickle Cell Disease 
Using the new technique of protein electrophoresis, Linus Pauling and colleagues showed
in 1949 that the hemoglobin from patients with sickle cell disease is different than that
of normals. This made sickle cell disease the first disorder in which an abnormality in a
protein was known to be at fault. 
Amino Acid Substitution in Sickle Hemoglobin 
In 1956, Vernon Ingram, then at the MRC in England, and J.A. Hunt sequenced sickle
hemoglobin and showed that a glutamic acid at position 6 was replaced by a valine in
sickle cell disease. Using the known information about amino acids and the codons that
coded for them, he was able to predict the mutation in sickle cell disease. This made
sickle cell disease the first known genetic disorder. 
Preventive Treatment for Sickle Cell Disease 
Hydroxyurea became the first (and only) drug proven to prevent complications of sickle
cell disease in the Multicenter Study of Hydroxyurea in Sickle Cell Anemia, which was
completed in 1995.
How Does Sickle Cell Cause Disease?
The Mutation in Hemoglobin
Sickle cell disease is a blood condition primarily affecting people of African ancestry.
The disorder is caused by a single change in the amino acid building blocks of the
oxygen-transport protein, hemoglobin. This protein, which is the component that makes red
cells red, has two subunits. The alpha subunit is normal in people with sickle cell
disease. The ?-subunit has the amino acid valine at position 6 instead of the glutamic
acid that is there normally. The alteration is the basis of all the problems that occur
in people with sickle cell disease. The schematic diagram shows the first eight-of the
146 amino acids in the ?-globin subunit of the hemoglobin molecule. The amino acids of
the hemoglobin protein are represented as a series of linked, colored boxes. The lavender
box represents the normal glutamic acid at position 6. The dark green box represents the
valine in sickle cell hemoglobin. The other amino acids in sickle and normal hemoglobin
are identical. 
The molecule, DNA (deoxyribonucleic acid), is the fundamental genetic material that
determines the arrangement of the amino acid building blocks in all proteins. Segments of
DNA that code for particular proteins are called genes. The gene that controls the
production of the ?-subunit of hemoglobin is located on one of the 46 human chromosomes
(chromosome #11). People have twenty-two identical chromosome pairs (the twenty-third
pair is the unlike X and Y-chromosomes that determine a person's sex). One of each pair
is inherited from the father, and one from the mother. Occasionally, a gene is altered in
the exchange between parent and offspring. This event, called mutation, occurs extremely
infrequently. Therefore, the inheritance of sickle cell disease depends totally on the
genes of the parents. 
If only one of the ?-globin genes is the sickle gene and the other is normal, the person
is a carrier. The condition is called sickle cell trait. With a few rare exceptions,
people with sickle cell trait are completely normal. If both ?-globin genes code for the
sickle protein, the person has sickle cell disease. Sickle cell disease is determined at
conception, when a person acquires his/her genes from the parents. Sickle cell disease
cannot be caught, acquired, or otherwise transmitted.
The hemoglobin molecule (made of alpha and ?-globin subunits) picks up oxygen in the
lungs and releases it when the red cells reach peripheral tissues, such as the muscles.
Ordinarily, the hemoglobin molecules exist as single, isolated units in the red cell,
whether they have oxygen bound or not. Normal red cells maintain a basic disc shape,
whether they are transporting oxygen or not.
The picture is different with sickle hemoglobin. Sickle hemoglobin exists as isolated
units in the red cells when they have oxygen bound. When sickle hemoglobin releases
oxygen in the peripheral tissues, however, the molecules tend to stick together and form
long chains or polymers. These polymers distort the cell and cause it to bend out of
shape. When the red cells return to the lungs and pick up oxygen again, the hemoglobin
molecules resume their solitary existence (the left of the diagram).
A single red cell may traverse the circulation four times in one minute. Sickle
hemoglobin undergoes repeated episodes of polymerization and depolymerization. This
Ping-Pong alteration in the state of the molecules damages the hemoglobin and ultimately
the red cell itself. 
Polymerized sickle hemoglobin does not form single strands. Instead, the molecules group
in long bundles of 14 strands each that twist in a regular fashion, much like a braid.
These bundles self-associate into even larger structures that stretch and distort the
cell. An analogy would be a water ballon that formed ice sickles that extended and
stretched the ballon. The stretching of the rubber of the ballon is similar to what
happens to the membrane of the red cell.
Despite their imposing appearance, the forces that hold these sickle hemoglobin polymers
together are very weak. The abnormal valine amino acid at position 6 in the ?-globin
chain interacts weakly with the ? globin chain in an adjacent sickle hemoglobin molecule.
The complex twisting, 14-strand structure of the bundles produces multiple interactions
and cross-interactions between molecules. On the other hand, the weak nature of the
interaction opens one strategy to treat sickle cell disease.
Some types of hemoglobin molecules, such as that found before birth (fetal hemoglobin),
block the interactions between the hemoglobin S molecules. All people have fetal
hemoglobin in their circulation before birth. Fetal hemoglobin protects the unborn and
newborns from the effects of sickle cell hemoglobin. Unfortunately, this hemoglobin
disappears within the first year after birth. One approach to treating sickle cell
disease is to rekindle production of fetal hemoglobin. The drug, Hydroxyurea induces
fetal hemoglobin production in some patients with sickle cell disease and improves the
clinical condition of some patients. 
The Sickle Red Cell
The schematic diagram shows the changes that occur as sickle or normal red cells release
oxygen in the microcirculation. The upper panel shows that normal red cells retain their
biconcave shape and move through the microcirculation (capillaries) without problem. In
contrast, the hemoglobin polymerizes in sickle red cells when they release oxygen, as
shown in the lower panel. The polymerization of hemoglobin deforms the red cells. The
problem, however, is not simply one of abnormal shape. The membranes of the cells are
rigid due in part to repeated episodes of hemoglobin polymerization/depolymerization as
the cells pick up and release oxygen in the circulation. These rigid cells fail to move
through the microcirculation, blocking local blood flow to a microscopic region of
tissue. Amplified many times, these episodes produce tissue hypoxia (low oxygen supply).
The result is pain, and often damage to organs. 
The damage to red cell membranes plays an important role in the development of
complications in sickle cell disease. Robert Hebbel at the University of Minnesota and
colleagues were among the first workers to show that the heme component of hemoglobin
tends to be released from the protein with repeated episodes of sickle hemoglobin
polymerization. Some of this free heme lodges in the red cell membrane. The iron in the
center of the heme molecule promotes formation of very dangerous compounds, called oxygen
radicals. These molecules damage both the lipid and protein components of the red cell
membrane. As a consequence, the membranes become stiff. Also, the damaged proteins tend
to clump together to form abnormal clusters in the red cell membrane. Antibodies develop
to these protein clusters, leading to even more red cell destruction (hemolysis).
Red cell destruction or hemolysis causes the anemia in sickle cell disease. The
production of red cells by the bone marrow increases dramatically, but is unable to keep
pace with the destruction. Red cell production increases by five to ten-fold in most
patients with sickle cell disease. The average half-life of normal red cells is about 40
days. In-patients with sickle cell disease, this value can fall to as low as four days.
The volume of active bone marrow is much expanded in-patients with sickle cell disease
relative to nomal in response to demands for higher red cell production. 
The degree of anemia varies widely between patients. In general, patients with sickle
cell disease have hematocrits that are roughly half the normal value (e.g., about 25%
compared to about 40-45% normally). Patients with hemoglobin SC disease (where one of the
?-globin genes codes for hemoglobin S and the other for the variant, hemoglobin C) have
higher hematocrits than do those with homozygous Hb SS disease. The hematocrits of
patients with Hb SC disease run in low- to mid-thirties. The hematocrit is normal for
people with sickle cell trait.
How Do People Get Sickle Cell Disease?
Sickle cell disease is an inherited condition. The genes received from one's parents
before birth determine whether a person will have sickle cell disease. Sickle cell
disease cannot be caught or passed on to another person. The severity of sickle cell
disease varies tremendously. Some people with sickle cell disease lead lives that are
nearly normal. Others are less fortunate, and can suffer from a variety of complications.

How Are Genes Inherited?
At the time of conception, a person receives one set of genes from the mother (egg) and a
corresponding set of genes from the father (sperm). The combined effects of many genes
determine some traits (hair color and height, for instance). One gene pair determines
other characteristics. Sickle cell disease is a condition that is determined by a single
pair of genes (one from each parent). 
Inheritance of Sickle Cell Disease
The genes are those which control the production of a protein in red cells called
hemoglobin. Hemoglobin binds oxygen in the lungs and delivers it to the peripheral
tissues, such as the liver. Most people have two normal genes for hemoglobin. Some people
carry one normal gene and one gene for sickle hemoglobin. This is called sickle cell
trait. These people are normal in almost all respects. Problems from the single sickle
cell gene develop only under very unusual conditions. 
People who inherit two genes for sickle hemoglobin (one from each parent) have sickle
cell disease. With a few exceptions, a child can inherit sickle cell disease only if both
parents have one gene for sickle cell hemoglobin. The most common situation in which this
occurs is when each parent has one sickle cell gene. In other words, each parent has
sickle cell trait. Figure 1 shows the possible combination of genes that can occur for
parents each of whom has sickle cell trait.
Figure 1. (ABOVE) Inheritance of sickle genes from parents with sickle cell trait. As
shown in the graphic, the couple has one chance in four that the child will be normal,
one chance in four that the child will have sickle cell disease, and one chance in two
that the child will have sickle cell trait.
A one-in-four chance exists that a child will inherit two normal genes from the parents.
A one-in-four chance also exists that a child will inherit two sickle cell genes, and
have sickle cell disease. A one-in-two chance exists that the child will inherit a normal
gene from one parent and a sickle gene from the other. This would produce sickle trait. 
These probabilities exist for each child independently of what happened with prior
children the couple may have had. In other words, each new child has a one-in-four chance
of having sickle cell disease. A couple with sickle cell trait can have eight children,
none of whom have two sickle genes. Another couple with sickle trait can have two
children each with sickle cell disease. The inheritance of sickle cell genes is purely a
matter of chance and cannot be altered. 
Do Factors Other Than Genes Influence Sickle Cell Disease?
Sickle cell disease is quite variable in itself. Other blood conditions can influence
sickle cell disease, however. One of the most important is thalassemia. One form of
thalassemia, called ?-thalassemia, reduces the production of normal hemoglobin. A person
with one normal hemoglobin gene and one thalassemia gene has thalassemia trait (also
called thalassemia minor). Parents who have sickle cell trait and thalassemia trait have
one chance in four of having a child with one gene for sickle cell disease and one gene
for ?-thalassemia (Figure 2). This condition is sickle ?-thalassemia. The severity
varies. Some patients with sickle ?-thalassemia have a condition as severe as sickle cell
disease itself. People of Mediterranean origin who have a sickle condition most often
have sickle ?-thalassemia.
Figure 2. (BELOW ON LAST PAGE) Inheritance of hemoglobin genes from parents with sickle
cell trait and thalassemia trait. As illustrated, the couple has one chance in four that
the child will have the genes both for sickle hemoglobin and for thalassemia. The child
would have sickle ?-thalassemia. The severity of this condition is quite variable. The
nature of the thalassemia gene (?o or ?+) greatly influences the clinical course of the
disorder.
Another disorder that interacts with sickle cell disease is hemoglobin SC disease. The
abnormal hemoglobin C gene is relatively harmless. Even people with two hemoglobin C
genes have a relatively mild clinical condition. When hemoglobin C combines with
hemoglobin S, the result is hemoglobin SC disease. On average, hemoglobin SC disease is
milder than sickle cell disease. However, some patients with hemoglobin SC disease have a
clinical condition as severe as any with sickle cell disease. The reason for the marked
variability in the clinical course of hemoglobin SC disease is unknown. We do know that
the tendency of hemoglobin C to produce red cell dehydration is a major reason that the
combination of hemoglobins S and C is so problematic. 
Figure 3. (ABOVE) Inheritance of hemoglobin genes from parents with sickle cell trait and
hemoglobin C trait. As illustrated, the couple has one chance in four that the child will
have the genes both for sickle hemoglobin and for hemoglobin C. The child would have
hemoglobin SC disease. Most patients with hemoglobin SC disease have a milder condition
than occurs with sickle cell disease (two sickle genes). Unfortunately, some patients run
a clinical course that is undistinguishable from sickle cell disease.
Are There Tests That Can Tell Me Whether I Have Sickle Cell Trait?
The answer is yes. Routine blood counts commonly performed in doctors' offices do not
give hints about the presence of sickle cell trait. The blood counts of most people with
sickle cell trait are normal. Only a special test, called a hemoglobin electrophoresis
indicates reliably whether a person has sickle trait. In addition, the hemoglobin
electrophoresis will detect hemoglobin C and ?-thalassemia. 
How Can I Be Tested for Sickle Cell Trait?
Most large hospitals and clinics can perform routine hemoglobin electrophoresis. Smaller
laboratories send the test to commercial firms for testing. If you are concerned about
the possibility of having sickle cell trait, you should speak with your doctor.
Overview
Everyone with sickle cell disease shares the same gene mutation. A thymine replaces an
adenine in the DNA encoding the ?-globin gene. Consequently, the amino acid valine
replaces glutamic acid at the sixth position in the ?-globin protein product. The change
produces a phenotypically recessive characteristic. Most commonly sickle cell disease
reflects the inheritance of two mutant alleles, one from each parent. 
The final product of this mutation, hemoglobin S is a protein whose quaternary structure
is a tetramer made up of two normal alpha-polypeptide chains and two aberrant
?s-polypeptide chains. The primary pathological process leading ultimately to sickle
shaped red blood cells involves this molecule. After deoxygenation of hemoglobin S
molecules, long helical polymers of HbS form through hydrophobic interactions between the
?s-6 valine of one tetramer and the ?-85 phenylalanine and ?-88 leucine of an adjacent
tetramer. 
Deformed, sickled red cells can occlude the microvascular circulation, producing vascular
damage, organ infarcts, painful crises and other such symptoms associated with sickle
cell disease. Although everyone with sickle cell disease shares a specific, invariant
genotypic mutation, the clinical variability in the pattern and severity of disease
manifestations is astounding. In other genetic disorders such as cystic fibrosis,
phenotypic variability between patients can be traced genotypic variability. Such is not
the case, however, with sickle cell disease. Physicians and researchers have sought
explanations of the variability associated with the clinical expression of this disease.
The most likely causes of this inconstancy are disease-modifying factors. I have reviewed
the role of some of these factors, and tried to ascertain the clinical importance of
each. 
Fetal Hemoglobin
Augmented post-natal expression of fetal hemoglobin is perhaps the most widely recognized
modulator of sickle cell disease severity. Fetal hemoglobin, as its name implies is the
primary hemoglobin present in the fetus from mid to late gestation. The protein is
composed of two alpha-subunits and two gamma-subunits. The gamma-subunit is a protein
product of the ?-gene cluster. Duplicate genes duplicate upstream of the ?-globin gene
encodes fetal globin. 
Fetal hemoglobin binds oxygen more tightly than does adult hemoglobin A. The
characteristic allows the developing fetus to extract oxygen from the mother's blood
supply. After birth, this trait is no longer necessary and the production of the
gamma-subunit decreases as the production of the ?-globin subunit increases. The ?-globin
subunit replaces the gamma-globin subunit in the hemoglobin tetramer so that eventually
adult hemoglobin replaces fetal hemoglobin as the primary component red cells. 
HbF levels stabilize during the first year of life, at less than 1% of the total
hemoglobin. In cases of hereditary persistence of fetal hemoglobin, that percentage is
much higher. This persistence substantially ameliorates sickle cell disease severity. 
Mechanism of Protection
Two properties of fetal hemoglobin help moderate the severity of sickle cell disease.
First, HbF molecules do not participate in the polymerization that occurs between
molecules of deoxyHbS. The gamma-chain lacks the valine at the sixth residue to interact
hydrophobically with HbS molecules. HbF has other sequence differences from HbS that
impede polymerization of deoxyHbS. Second, higher concentrations of HbF in a cell infer
lower concentrations of HbS. Polymer formation depends exponentially on the concentration
of deoxyHbS. Each of these effects reduces the number of irreversibly sickle cells (ISC).

Hemoglobin F Levels and Amelioration of Sickle Cell Disease
The level of HbF needed to benefit people with sickle cell disease is a key question to
which different studies supply varying answers. Bailey examined the correlation between
early manifestation of sickle cell disease and fetal hemoglobin level in Jamaicans. They
concluded that moderate to high levels of fetal hemoglobin (5.4-9.7% to 39.8%) reduced
the risk for early onset of dactylics, painful crises, acute chest syndrome, and acute
splenic sequestration. 
Platt examined predictive factors for life expectancy and risk factors for early death
(among Black Americans). In their study, a high level of fetal hemoglobin (*8.6%) augured
improved survival. Koshy et al. reported that fetal hemoglobin levels above 10% were
associated with fewer chronic leg ulcers in American children with sickle cell disease. 
Other studies, however, suggest that protection from the ravages of sickle cell disease
occur only with higher levels of HbF. In a comparison of data from Saudi Arabs and
information from Jamaicans and Black Americans, Perrine et al. found that serious
complications occurred only 6% to 25% as frequently in Saudi Arabs as North American
Blacks. In addition mortality under the age of 15 was 10% as great among Saudi Arabs.
Further, these patients experienced no leg ulcers, reticulocyte counts were lower and
hemoglobin levels were higher on average. 
The average a fetal hemoglobin level in the Saudi patients ranged between 22-26.8%. This
is more than twice that reported in studies mentioned above. Kar et al. compared patients
from Orissa State, India to Jamaican patients with sickle cell. These patients also had a
more benign course when compared with Jamaican patients. The reported protective level of
fetal hemoglobin in this study was on average 16.64%, with a range of 4.6% to 31.5%.
Interestingly, ?-globin locus haplotype analysis shows that the Saudi HbS gene and that
in India have a common origin (see below). 
These studies suggest that the level of fetal hemoglobin that protects against the
complications of sickle cell disease depend strongly on the population group in question.
Among North American blacks, fetal hemoglobin levels in the 10% range ameliorate disease
severity. The higher average level of fetal hemoglobin could contribute to the generally
less severe disease in Indians and Arabs. 
Another study that suggests only a small role at best for fetal hemoglobin as a modifier
of sickle cell disease severity was reported by El-Hazmi. The subjects were Saudi Arabs
in whom a variety of symptoms associated with sickle cell disease were assessed to form a
severity index. The author concluded that among his patients, no correlation existed
between HbF and the severity index. 
However, his analysis has a fundamental flaw. El-Hazmi failed to examine the effect of
HbF on each of these symptoms individually. Their important information and an
association between fetal hemoglobin levels specific disease manifestations could be
concealed in his data. However, the study reinforces the conclusion that fetal hemoglobin
levels most likely work in conjunction with other moderating factors to determine
clinical severity in-patients with sickle cell disease. 
Alpha-Thalassemia
Concurrent alpha-thalassemia has also been examined as a modifier of sickle cell disease
severity. Alpha-thalassemia, like sickle cell disease, is a genetically inherited
condition. The loss of one or more of the four genes encoding the alpha globin chain (two
each on chromosome 16) produces alpha-thalassemia. A gene deletion most commonly is at
fault. The deletion results from unequal crossover between adjacent alpha-globin genes
during the prophase I of meiosis I. Such a crossover leaves one gamete with one
alpha-gene and the other gamete with three alpha genes. Upon fertilization the zygote can
have 2, 3, 4, or 5 alpha genes depending on the make up of the other parental gamete. In
people of African descent, the most common haploid gamete of this type is alpha-thal-2 in
which there is one deletion on each of the number 16 chromosomes in the patient.
Heterozygotes for this allele, therefore, have three alpha genes (one alpha gene on one
of the number 16 chromosomes, two alpha genes on the other). 
Embury et al. (1984) examined the effect of concurrent alpha-thalassemia and sickle cell
disease. Based on prior studies, they proposed that alpha-thalassemia reduces
intraerythrocyte HbS concentration, with a consequent reduction in polymerization of
deoxyHbS and hemolysis. They investigated the effect of alpha gene number on properties
of sickle erythrocytes important to the hemolytic and rheological consequences of sickle
cell disease. Specifically they looked for correlations between the alpha gene number and
irreversibly sickled cells, the fraction of red cells with a high hemoglobin
concentration (dense cells), and red cells with reduced deformabilty. 
The investigators found a direct correlation between the number of alpha-globin genes and
each of these indices. A primary effect of alpha-thalassemia was reduction in the
fraction of red blood cells that attained a high hemoglobin concentration. These dense
cells result from potassium loss due to acquired membrane leaks. The overall
deformability of dense RBCs is substantially lower than normal. 
This property of alpha-thalassemia was confirmed by comparison of red cells in people
with or without 2-gene deletion alpha-thalassemia (and no sickle cell genes). The cells
in the nonthalassemic individuals were denser than those from people with 2-gene deletion
alpha-thalassemia. The difference in median red cell density produced by
alpha-thalassemia was much greater in-patients sickle cell disease. 
Reduction in overall hemoglobin concentration due to absent alpha genes is not the only
mechanism by which alpha-thalassemia reduces the formation of dense and irreversibly
sickled cells. In reviewing the available literature, Embry and Steinburg suggested that
alpha-thalassemia moderate's red cell damage by increasing cell membrane redundancy. This
protects against sickling-induced stretching of the cell membrane. Potassium leakage and
cell dehydration would be minimized. 
These two papers by Embury et al. give some insight into the moderation of sickle cell
disease severity by alpha thalassemia. Some deficiencies exist, nonetheless. The first
paper makes no mention of the patient pool. Unspecified are the number of patients used,
their ethnicity, or their state of health when blood samples were taken. This information
would help establish the statistical reliability of the data, and its applicability
across patient groups. Despite these limitation, the work provides important insight into
the mechanisms by which alpha-thalassemia ameliorates sickle cell disease severity. 
Ballas et al reached different conclusions regarding alpha thalassemia and sickle cell
disease than did Embury et al . They reported that decreased red blood cell deformability
was associated with reduced clinical severity of sickle cell disease.
Patients with more highly deformabile red cells had more frequent crises. They also found
that fewer dense cells and irreversible sickle cells correlated inversely with the
severity of painful crises. Like Embury et al., Ballas and colleagues found alpha
thalassemia was associated with fewer dense red cells. 
In addition, Ballas' group found that alpha thalassemia was associated with less severe
hemolysis. However they reached no clear conclusion concerning alpha gene number and
deformability of RBC except to note that the alpha thalassemia was associated with less
red cell dehydration. 
The two studies are not completely at odds. Both state that concurrent alpha-thalassemia
reduces hemolytic anemia. They agree that this occurs through reduction in the number of
dense cells, a number directly related to the fraction of irreversibly sickled cells.
Embury et al. concludes that through this mechanism red blood cell deformability is
increased. 
The investigators diverge, however, on the relationship to clinical severity of dense
cells and rigid cells. Ballas et al. asserts that both the reduction of dense cells and
rigid cells contribute to disease severity. They advance three possible