Sickle Cell Disease

Slide 1 · Sickle Cell Disease

Inherited hemoglobinopathy: molecular mechanism, morphology and clinicopathologic consequences

Dr Abu Suraih Sakhri

MBBS, MD (Pathology)

www.abusuraihsakhri.com

Slide 2 · Learning objectives

Trace the β-globin mutation to HbS polymerization and red-cell injury.

Explain why hypoxia, dehydration, acidosis and slowed flow accentuate sickling.

Recognize characteristic blood, marrow and splenic pathology.

Relate hemolysis and vaso-occlusion to acute crises and chronic organ damage.

Connect diagnostic tests and disease-modifying treatment to their mechanistic targets.

Slide 3 · One mutation, two major injury pathways

Deoxygenated HbS polymerizes within red cells

Cumulative membrane injury

Premature red-cell destruction

Chronic hemolytic anemia

Adhesion and vascular stagnation

Microvascular occlusion

Ischemia and tissue infarction

Both pathways arise from abnormal HbS behavior but do not track perfectly together.

Slide 4 · Hemoglobin is a four-chain molecule

Normal adult hemoglobins

Disease-associated form

HbA: α₂β₂ — predominant adult hemoglobin

HbA₂: α₂δ₂ — minor adult fraction

HbF: α₂γ₂ — fetal hemoglobin

HbS: α₂βS₂ in homozygous sickle cell anemia

The mutation changes β-globin properties, not the basic tetrameric architecture

Mixtures of hemoglobins influence sickling behavior

Slide 5 · A missense mutation changes β-globin

Point mutation in β-globin gene

Glutamate replaced by valine

Hydrophobic region in HbS

Abnormal behavior on deoxygenation

The molecular substitution alters physicochemical interactions between hemoglobin molecules.

Slide 6 · HbS differs because valine is hydrophobic

Normal β-globin residue

Mutant βS-globin residue

Glutamate carries a charged side chain.

Surface interactions maintain soluble hemoglobin behavior.

Valine has a hydrophobic side chain.

Deoxygenation facilitates inappropriate HbS association.

Higher-order HbS polymers distort the red cell.

Slide 7 · Inheritance: Two HbAS parents

Gametes: A or S from each heterozygous parent

AA

AS

AS

SS

A

S

A

S

Each pregnancy has a one-in-four chance of HbSS, a two-in-four chance of HbAS, and a one-in-four chance of HbAA.

Slide 8 · Sickle cell trait is not sickle cell anemia

HbAS: Sickle cell trait

HbSS: Sickle cell anemia

One sickle allele; HbA plus HbS.

Most red cells resist sickling under usual oxygen conditions.

Prolonged severe hypoxia may elicit sickling.

Two sickle alleles; HbS predominates.

Polymerization occurs with ordinary physiologic deoxygenation.

Chronic hemolysis and clinically important vascular injury result.

Slide 9 · Hemoglobin mixtures modify disease severity

Hemoglobin context

Key molecular property

Clinicopathologic implication

HbAS

HbA interferes with HbS assembly

Mostly asymptomatic trait

HbSS

Predominantly HbS

Sickling disease

HbSC

HbS plus HbC; Cell dehydration

Symptomatic, often less severe than HbSS

HbF-rich cells

HbF strongly inhibits HbS polymerization

Lower sickling tendency

Slide 10 · Malaria drove positive selection for HbAS

Falciparum malaria in endemic regions

Selective advantage of HbAS children

Lower severe malaria and death risk

Higher HbS allele frequency

A population-genetic explanation for the distribution of HbS, not a benefit of HbSS disease.

Slide 11 · Malaria protection: Enhanced red-cell clearance

Parasite consumes oxygen and lowers intracellular pH

Infected HbAS cell more likely to sickle

Splenic / hepatic phagocytic removal

Reduced circulating parasite burden

Proposed mechanism; Mechanistic details are not fully established.

Slide 12 · Malaria protection: Impaired adhesion

Intracellular sickling in infected HbAS cells

Reduced parasite-induced membrane knobs

Impaired PfEMP-1–associated adhesion

Reduced severe malaria pathogenesis

Proposed mechanism; Parasite PfEMP-1 is involved in endothelial adhesion.

Slide 13 · Deoxygenation is the polymerization switch

Oxygenated | deoxygenated

Schematic

Oxygenated HbS remains more soluble.

Deoxygenated HbS has increased tendency to polymerize.

Reoxygenation may reverse early, not yet fixed, shape change.

Schematic cell silhouettes — not a peripheral blood smear.

Slide 14 · Molecular assembly progresses to long fibers

Deoxygenated HbS molecules associate

Cytosol becomes viscous / gel-like

Needlelike intracellular polymers form

Red cells become distorted and rigid

A time-dependent process: Sickling depends on both kinetics and microvascular transit.

Slide 15 · Sickling is governed by local conditions

Variable

Promotes sickling

Reduces sickling

Oxygenation

Deoxygenation

Reoxygenation before irreversible injury

Cell Hb concentration

Dehydration / higher MCHC

Lower HbS concentration

Intracellular pH

Reduced pH

Less acidic conditions

Circulatory transit

Slow flow / prolonged deoxygenation

Fast transit

Hb composition

Higher effective HbS fraction

HbA or especially HbF

Slide 16 · HbA delays polymer growth in HbAS cells

HbAS red cells

HbSS red cells

Approximately 40% HbS’ description.

The remaining HbA interferes with HbS polymerization.

Prolonged, relatively severe hypoxia is usually needed for sickling.

Almost all hemoglobin is HbS.

The inhibitory effect of HbA is absent.

Sickling becomes important at typical tissue oxygen tensions.

Slide 17 · HbF is a strong inhibitor of HbS polymers

Increased intracellular HbF

Less HbS polymerization during deoxygenation

Reduced red-cell distortion

Less hemolytic and vaso-occlusive pathology

High HbF expression is a major naturally occurring and therapeutic modifier.

Slide 18 · Why symptoms often begin after infancy

Newborn

High fetal hemoglobin

Early infancy

Physiologic HbF decline

~5–6 months

Clinical manifestations may emerge

Later life

Variable cumulative tissue damage

Timing for HbSS; Onset and severity vary among individuals.

Slide 19 · Persistent fetal hemoglobin can attenuate disease

Typical HbF decline

Persistently high HbF

HbF falls during infancy.

HbS becomes the major hemoglobin species.

Vaso-occlusive and hemolytic manifestations appear.

HbF interferes strongly with HbS assembly.

Sickling frequency and severity may decline.

This illustrates a genetic modifier of phenotype.

Slide 20 · HbSC is a distinct sickling phenotype

HbAS trait

Compound HbSC

HbS diluted by a larger HbA component.

Sickling usually requires severe or prolonged hypoxia.

HbS constitutes a larger share of total hemoglobin.

Aging red cells lose salts and water.

The resultant HbS concentration permits symptomatic disease.

Slide 21 · Cell dehydration amplifies HbS polymerization

Loss of K⁺ and water

Increased mean corpuscular Hb concentration

Greater intracellular HbS concentration

Faster polymer formation during deoxygenation

HbS concentration strongly affects the probability of aggregation.

Slide 22 · Coexistent α-thalassemia may lessen sickling

Reduced α-globin synthesis

Lower total cell hemoglobin concentration

Lower probability of HbS polymerization

Milder clinical expression may result

The effect of accompanying α-thalassemia in HbSS.

Slide 23 · Acidosis favors deoxygenated HbS

Intracellular pH decreases

Hemoglobin oxygen affinity falls

Fraction of deoxygenated HbS rises

Sickling tendency increases

The effect is mediated partly by the oxygenation state of HbS.

Slide 24 · Transit time determines where cells sickle

Rapid microvascular transit

Slow microvascular transit

Deoxygenated exposure may be brief.

Polymerization may not reach a critical threshold before cells exit.

Major occlusion is less likely under ordinary flow.

Red cells remain deoxygenated longer.

HbS polymers can assemble during transit.

Rigid cells arrest and reinforce local obstruction.

Slide 25 · The spleen and marrow are high-risk beds

Physiologically sluggish tissue blood flow

Longer residence in low-oxygen microvasculature

Intracellular HbS polymer formation

Local sequestration and ischemic injury

Normal slow transit in the spleen and bone marrow.

Slide 26 · Inflamed vessels become more permissive to stasis

Inflammatory vasodilation and vascular leak

Sluggish microvascular transit

Longer tissue deoxygenation

Sickling, adhesion and local occlusion

Inflammation also enhances endothelial adhesion molecule expression.

Slide 27 · Repeated sickling injures the cell membrane

HbS fibers mechanically deform the cell

Membrane skeleton is disrupted

Calcium influx and ion channel activation

Salt and water loss; Greater rigidity

Membrane injury persists when an early sickling episode reverses.

Slide 28 · A dehydration–rigidity feedback loop

Sickling

Membrane damage

K⁺ / H₂O loss

Higher HbS concentration

Recurrent episodes favor dense, poorly deformable erythrocytes.

Slide 29 · Reversible and irreversible sickling differ

Earlier | permanently damaged

Schematic

Early distortions may reverse on oxygenation.

Repeated cycles create cumulative, permanent membrane damage.

Some cells remain sickled even when oxygenated.

Schematic silhouettes — irreversibility refers to sustained cell shape and stiffness.

Slide 30 · Sickle cells are removed through two routes

Sickled cells have reduced survival

Poor deformability traps cells in phagocytic organs.

Mononuclear phagocytes remove severely damaged cells.

Extravascular hemolysis predominates.

Sickled cells are mechanically fragile.

Some rupture within the circulation.

Intravascular hemolysis also occurs.

The proportion of irreversibly sickled cells correlates with severity of hemolysis.

Slide 31 · Hemolysis and vaso-occlusion are not identical

Chronic hemolysis

Microvascular occlusion

Tracks cumulative membrane injury and irreversible sickling.

Produces anemia, bilirubin generation and free plasma hemoglobin.

Depends also on red-cell adhesion and vascular conditions.

Not simply proportional to irreversibly sickled cell count.

Produces tissue hypoxia, pain and infarction.

Slide 32 · Adhesion links inflammation to occlusion

Inflammatory leukocytes release mediators

Endothelial adhesion molecules increase

Adherent sickle red cells arrest

Prolonged hypoxia deepens vaso-occlusion

Adhesion is a dynamic process involving sickle cells and activated endothelium.

Slide 33 · Free hemoglobin may deplete nitric oxide

Intravascular sickle-cell lysis

Free hemoglobin binds / inactivates NO

Vascular tone and platelet aggregation increase

Stasis, thrombosis and sickling may worsen

Nitric-oxide depletion as a possible contributor, not the sole cause of occlusion.

Slide 34 · Microvascular occlusion creates a vicious cycle

Adhesion / slowed flow

Local hypoxia

HbS sickling

Microvascular obstruction

Each turn can increase deoxygenation and promote further sickling.

Slide 35 · Two parallel paths explain the core phenotype

HbS polymerization in deoxygenated cells

Membrane injury → shortened red-cell survival

Anemia → increased marrow erythropoiesis

Heme turnover → jaundice / pigment stones

Adhesive rigid cells → obstruction

Infarction → painful crises

Chronic ischemia → multisystem injury

The paths interact through inflammation, free hemoglobin and tissue oxygen delivery.

Slide 36 · Peripheral blood: Characteristic cell patterns

Cell silhouettes (schematic)

Irreversibly sickled cells have elongated curved or pointed contours.

Target cells accompany altered red-cell hydration.

Anisocytosis and poikilocytosis are present.

Reticulocytosis signals compensatory erythropoiesis.

Original schematic of cell shapes; Not authentic patient microscopy.

Slide 37 · Howell–Jolly bodies signal loss of splenic function

Red cell | nuclear remnant

Schematic

Small nuclear remnants may persist within circulating erythrocytes.

Normally the spleen removes these inclusions.

Their appearance in sickle cell disease supports functional asplenia.

Schematic depiction of a red cell with an inclusion; Not a micrograph.

Slide 38 · Marrow expands to compensate for hemolysis

Persistent shortened erythrocyte survival

Increased erythropoietic demand

Marrow erythroid hyperplasia

Expansion of hematopoietic spaces

Extramedullary hematopoiesis may develop when compensatory demand is substantial.

Slide 39 · Marrow expansion remodels the skeleton

Chronic erythroid hyperplasia

Expansion and remodeling of bony marrow spaces

Resorption plus reactive bone formation

Characteristic skull and facial skeletal changes

Prominent cheekbones and a “crewcut” skull appearance.

Slide 40 · Hemoglobin turnover produces pigment stones

Premature erythrocyte destruction

Increased heme breakdown

Hyperbilirubinemia

Pigment gallstone formation

This is a consequence of sustained hemolysis rather than microvascular infarction.

Slide 41 · Early spleen: Sequestration and congestion

Sickled cells lodge in red pulp cords and sinusoids

Congestion causes splenic enlargement

Poor blood flow impairs filtration

Children may have functional hyposplenism

Pronounced early red-pulp congestion and trapping.

Slide 42 · Repeated infarction leads to autosplenectomy

Childhood

Congested, enlarged spleen

Repeated events

Ischemic infarctions

Progression

Fibrosis and shrinkage

Older patients

Small fibrotic remnant

Functional loss may precede near-complete anatomic involution.

Slide 43 · Splenic destruction changes infection risk

Progressive loss of splenic filtration

Reduced clearance of organisms from blood

Impaired defense against encapsulated bacteria

Howell–Jolly bodies persist

Risk of serious bacterial sepsis and meningitis

Source additionally notes impaired alternative-pathway opsonization of uncertain origin.

Slide 44 · Infarction is a systemic process

Affected site

Dominant injury

Potential consequence

Bone / marrow

Microvascular infarction

Severe painful bone crisis

Brain

Ischemic vascular injury

Stroke

Kidney

Medullary sickling / injury

Concentrating defect

Spleen

Recurrent infarction

Functional / anatomic asplenia

Retina

Vascular obstruction

Visual impairment

Pulmonary circulation

Vascular disease

Respiratory dysfunction

Slide 45 · Chronic leg ulcers reflect local stasis

Sickle-cell vascular stagnation

Persistent subcutaneous ischemia

Impaired tissue integrity

Chronic leg ulceration

Ulcers particularly in adults; They are uncommon in children.

Slide 46 · Baseline phenotype: Chronic hemolytic anemia

Hematocrit commonly around 18%–30% in sickle cell anemia.

Reticulocytosis reflects persistent marrow compensation.

Unconjugated bilirubin generation explains chronic jaundice and pigment stones.

Irreversibly sickled cells may be visible on the peripheral blood film.

Slide 47 · Acute crises have different mechanisms

Crisis

Immediate process

Characteristic pattern

Vaso-occlusive

Microvascular obstruction / infarction

Regional severe pain

Acute chest

Pulmonary inflammation + sickling

Fever, cough, pain, infiltrates

Splenic sequestration

Rapid trapping of blood in spleen

Enlarging spleen, hypovolemia

Aplastic

Parvovirus B19 suppresses erythropoiesis

Abrupt anemia with impaired marrow output

Slide 48 · Pain crises are episodes of tissue infarction

Vascular adhesion / sickling

Local obstruction

Hypoxic tissue injury

Severe regional pain

Infection, dehydration and acidosis can precipitate crises, but an obvious trigger is often absent.

Slide 49 · Hand–foot syndrome is a pediatric manifestation

What occurs

Diagnostic implication

Acute vaso-occlusive bone injury in the hands and feet.

Pain and swelling constitute dactylitis.

Often occurs in childhood.

Bone infarction can mimic acute osteomyelitis.

Clinical context and investigation are needed to distinguish them.

Do not infer infection merely from painful swelling.

Slide 50 · Acute chest syndrome is pulmonary vaso-occlusion

Pulmonary inflammation / infection

Slowed capillary transit and sickling

Pulmonary vascular obstruction

Hypoxemia and lung infiltrates

Typical features: Fever, cough, chest pain and pulmonary infiltrates.

Slide 51 · Acute chest syndrome can self-amplify

Pulmonary vaso-occlusion

Reduced oxygenation

More deoxygenated HbS

Additional pulmonary sickling

This feedback can become rapidly life-threatening; Exchange transfusion may be needed.

Slide 52 · Stroke reflects cerebral vascular injury

Vascular obstruction compromises brain oxygenation

Sickle red-cell adhesion

Endothelial vascular interactions

Proposed NO depletion

Vasoconstriction and ischemia

The relative contribution of these factors as proposed mechanisms.

Slide 53 · Retinopathy can threaten sight

Sickle-cell vascular obstruction

Retinal ischemic damage

Retinopathy

Reduced acuity or blindness

The organ-level manifestation derives from the same occlusive vascular pathology.

Slide 54 · Priapism is a vaso-occlusive complication

Penile vaso-occlusion

Persistent painful erection

Tissue hypoxia

Possible erectile dysfunction

The complication in males after puberty.

Slide 55 · Splenic sequestration can cause shock

Residual enlarged spleen in childhood

Sudden trapping of sickled cells

Rapid splenic enlargement and hypovolemia

Hemodynamic collapse may occur

This is mechanistically different from gradual autosplenectomy.

Slide 56 · Parvovirus B19 causes aplastic crisis

Parvovirus B19 infects erythroid progenitors

Erythropoiesis temporarily ceases

Ongoing red-cell destruction continues

Abrupt worsening of anemia

The failure of compensatory production, rather than increased sickling alone, explains the acute drop.

Slide 57 · Different crises demand different reasoning

Sudden severe pain / hypoxemia

Sudden anemia

Consider tissue ischemia and vaso-occlusion.

A new pulmonary infiltrate and respiratory signs indicate acute chest syndrome.

Enlarging spleen suggests sequestration.

Suppressed erythropoiesis suggests aplastic crisis.

Same underlying disease; Different proximate mechanism.

Slide 58 · Renal medulla promotes local sickling

Hypertonic renal medullary environment

Enhanced erythrocyte dehydration / sickling

Progressive medullary tissue injury

Impaired urinary concentration (hyposthenuria)

Urinary concentrating defects aggravate dehydration risk.

Slide 59 · Hyposthenuria increases vulnerability to crises

Medullary injury

Hyposthenuria

Predisposition to dehydration

Increased sickling tendency

A physiologic feedback linking kidney pathology to subsequent vaso-occlusive events.

Slide 60 · Chronic hypoxia affects multiple organs

Organ system

Mechanism or lesion

Clinical implication

Growth / development

Sustained anemia and tissue hypoxia

Impaired growth

Spleen

Congestion, infarction, fibrosis

Asplenia

Heart

Chronic hypoxic stress

Cardiac damage

Kidney

Medullary sickling

Hyposthenuria

Lungs

Chronic vascular / hypoxic injury

Pulmonary dysfunction

Slide 61 · Asplenia predisposes to encapsulated infection

Functional / anatomic splenic loss

Reduced filtering of blood-borne bacteria

Less effective clearance and opsonization

Sepsis and meningitis risk increases

Robbins specifically highlights pneumococcus, H. Influenzae and meningococcus.

Slide 62 · Prevention targets infectious vulnerability

Mechanistic vulnerability

Measures

Progressive loss of splenic tissue and function.

Impaired antibacterial defense, especially during childhood.

Vaccination against key pathogens.

Prophylactic antibiotics in appropriate patients.

Both help lower serious infection incidence.

Slide 63 · Clinical expression varies considerably

Milder course

More severe course

Fewer crises and less cumulative tissue damage.

Modifier hemoglobins may alter polymerization.

Frequent vaso-occlusive episodes.

Progressive organ impairment.

Multiple genetic and environmental modifiers are suspected.

Slide 64 · Diagnosis combines phenotype and Hb testing

Compatible clinical manifestations

Blood-film signs and reticulocytosis

Testing that identifies sickle hemoglobin

Confirmation of disease genotype / hemoglobin pattern

Morphology suggests the disorder; Specific Hb testing establishes its presence.

Slide 65 · Blood-film findings are supportive, not sufficient

Peripheral morphology

Required confirmation

Irreversibly sickled cells are distinctive.

Target cells and reticulocytosis support hemolysis.

Howell–Jolly bodies suggest asplenia.

Demonstrate sickle hemoglobin by appropriate testing.

Distinguish HbSS, HbAS and other sickle-containing patterns.

Account for phenotype variability.

Slide 66 · Newborn and prenatal testing identify HbS early

Newborn screening

Prenatal assessment

Routine U.S. Heel-stick screening.

Detects sickle hemoglobin before typical symptomatic age.

Provides time to plan prevention and surveillance.

Fetal DNA may be assessed from prenatal sampling.

Chorionic biopsy or amniocentesis are mentioned.

Family genotype informs reproductive counseling.

Slide 67 · Hydroxyurea modifies the disease mechanism

Hydroxyurea, an inhibitor of DNA synthesis

Raises red-cell HbF levels

HbF inhibits HbS polymerization

Reduces sickling burden

Reduces leukocyte production

Provides anti-inflammatory effects

Lessens vaso-occlusive crises

The precise HbF induction mechanism is not known.

Slide 68 · L-glutamine is described as an adjunct

L-glutamine added to hydroxyurea

Possible change in cellular metabolism

Possible reduction in oxidant stress

Fewer painful crises in treated patients

The proposed mechanism is uncertain in the Robbins section; This is not a dosing recommendation.

Slide 69 · Stem-cell transplantation offers curative potential

Hematopoietic stem-cell transplantation

Replacement of defective erythropoiesis

Production of red cells without the same HbS disease burden

Potential cure in selected patients

Transplantation for disease not adequately controlled by other treatment.

Slide 70 · Gene editing can reactivate fetal hemoglobin

CRISPR-based gene editing approach

Increased fetal hemoglobin production

Suppression of HbS polymerization

Potentially curative therapeutic strategy

Robbins 11th ed. Notes an approved gene-editing approach and substantial cost/access barriers.

Slide 71 · Therapeutic targets follow pathophysiology

Intervention

Main mechanism

Intended disease impact

Hydroxyurea

More HbF; Fewer leukocytes

Fewer vaso-occlusive episodes

L-glutamine

Possible lower oxidant stress

Fewer painful episodes

Stem-cell transplantation

Reconstitutes hematopoiesis

Potential cure

Gene editing

Raises HbF expression

Prevents HbS sickling / potential cure

Vaccines / antibiotics

Reduces infection vulnerability

Prevents serious infection

Slide 72 · Teaching case: Painful swollen hands

Clinical teaching scenario

A young child with known sickle cell disease develops acutely painful swelling of both hands and feet. The history does not establish an obvious precipitant.

Question for discussion

Which pathologic mechanism best explains this presentation, and what important clinical mimic should be considered?

Slide 73 · Case analysis: Hand–foot syndrome

Mechanism

Critical differential

Dactylitis is a childhood bone vaso-occlusive episode.

HbS polymerization contributes to microvascular stasis and infarction.

Acute osteomyelitis can resemble a bone crisis.

Pain alone does not determine whether the lesion is infectious.

Slide 74 · Teaching case: Sudden worsening of anemia

Clinical teaching scenario

A child with chronic HbSS-associated anemia develops a rapid decline in red-cell mass during an intercurrent viral illness, without a new focal painful infarction.

Question for discussion

Which crisis arises when erythroid progenitor production temporarily stops? How does it differ from splenic sequestration?

Slide 75 · Case analysis: Production versus sequestration

Aplastic crisis

Sequestration crisis

Parvovirus B19 suppresses erythroid progenitors.

Low production cannot offset the shortened sickle-cell life span.

Circulating erythrocytes rapidly pool in an enlarged spleen.

Hypovolemia and abrupt splenic enlargement are characteristic.

Slide 76 · Mechanism review: Trace the causal sequence

β-globin glutamate → valine

Deoxygenated HbS polymers

Rigid, adhesive, dehydrated red cells

Hemolysis plus vaso-occlusive ischemia

Use this sequence to explain morphology, clinical manifestations and therapeutic targets.

Slide 77 · Clinicopathologic review: Three patterns

Pattern

Recognize

Underlying mechanism

Chronic hemolysis

Anemia, reticulocytosis, pigment stones

Membrane damage and early RBC removal

Acute ischemia

Pain crisis, chest syndrome, stroke

Slowed flow, adhesion, sickling

Splenic dysfunction

Howell–Jolly bodies, infection risk

Congestion, infarction and fibrosis

Slide 78 · References and source boundaries

Robbins, Cotran & Kumar. Pathologic Basis of Disease. 11th ed. (2026), Chapter 14, “Sickle Cell Disease,” printed pp. 582–585.

Primary pathology content and clinical examples are derived from the supplied Robbins section.

Original editable diagrams are mechanistic schematics, not reproduced textbook figures.

Authentic textbook blood-smear and splenic photomicrographs were not embedded because source image files / reuse clearance were unavailable.

Current treatment protocols, dose regimens and eligibility criteria are intentionally outside this textbook-based lecture.