Why homozygotes of sickle cell not protected

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Here is a thorough explanation of why homozygotes (HbSS) are NOT protected against malaria, while heterozygotes (HbAS) are:

Why HbSS Homozygotes Are Not Protected Against Malaria

1. The Core Biochemical Difference

In heterozygotes (HbAS - sickle cell trait), red blood cells contain a mixture of normal HbA and sickle HbS. Under normal oxygen levels, these cells function adequately. But when Plasmodium falciparum infects these cells and consumes oxygen, the drop in intracellular pO2 and pH triggers selective sickling - causing the infected cells to be rapidly cleared by the spleen and liver before the parasite can complete its intracellular life cycle.

In homozygotes (HbSS), ALL hemoglobin is HbS. There is no normal HbA. Red blood cells sickle constantly, even without malarial infection, at normal oxygen tensions. This means:

The sickling in HbSS is not selective for infected cells - it affects all red cells
The cells that sickle include both infected and uninfected ones
The mechanism that makes sickle cells "recognizable targets" for immune clearance loses its specificity

2. Two Mechanisms That Explain Heterozygote Protection (Both Fail in HbSS)

From Robbins & Cotran Pathologic Basis of Disease:

Mechanism A - Accelerated clearance of infected cells:

"Metabolically active intracellular parasites consume oxygen and decrease intracellular pH, both of which promote sickling of HbAS red cells. These distorted, stiff cells may be cleared more rapidly by splenic and hepatic phagocytes, keeping parasite loads low."

In HbSS patients, ALL cells are already prone to clearance and destruction - this selective advantage is gone, replaced by generalized hemolytic anemia.

Mechanism B - Impaired PfEMP-1 knob formation (prevents cerebral malaria):

"Sickling also impairs the formation of membrane knobs containing a protein made by the parasite called PfEMP-1. These membrane knobs are implicated in adhesion of infected red cells to endothelium, which is believed to have an important pathogenic role in the most severe form of the disease, cerebral malaria."

While this could theoretically still operate in HbSS, the patients are so severely ill from the disease itself that malaria compounds an already catastrophic clinical picture.

3. HbSS Disease Is Itself Lethal - It Negates Any Selective Advantage

From Basic Medical Biochemistry (Lieberman & Peet):

"In Will S. and other homozygous individuals with sickle cell anemia, the red blood cells sickle more frequently than in heterozygotes, especially under conditions of low oxygen tension. The result is a vaso-occlusive crisis... the sickle cell allele is of little advantage to homozygous individuals."

The HbSS phenotype causes:

Vaso-occlusive crises - sickled cells clog capillaries causing ischemia and pain
Severe chronic hemolytic anemia - spleen destroys sickled cells constantly
Immunocompromise - functional asplenia (spleen is destroyed by repeated infarcts), leaving patients paradoxically MORE vulnerable to infections including encapsulated bacteria
Multi-organ damage - kidneys, lungs, brain, bone

Without modern medical care, HbSS patients historically had significantly reduced survival, meaning they did not gain a net selective fitness advantage from any partial malaria protection.

4. The Evolutionary (Population Genetics) Perspective

From Lippincott Biochemistry:

"The high frequency of the βS mutation among black Africans, despite its damaging effects in the homozygous state, suggests that a selective advantage exists for heterozygous individuals."

This is classic heterozygote advantage (overdominance):

Genotype	HbA/HbS ratio	Malaria risk	Sickle disease
HbAA (normal)	All HbA	High (no protection)	None
HbAS (trait)	Mixed	Low (PROTECTED)	Mild/none
HbSS (disease)	All HbS	Moderate-high	Severe/lethal

The HbAS heterozygote sits at the "sweet spot" - enough HbS to trigger selective sickling of parasite-infected cells, but enough HbA to keep the overall red cell pool functional. HbSS has crossed past that point: the disease burden (anemia, vaso-occlusion, organ failure) overwhelms any marginal protection against malaria.

This heterozygote advantage explains why the HbS allele persists at high frequencies in malaria-endemic regions (up to 25-40% in parts of sub-Saharan Africa) despite causing a lethal homozygous disease - as explained in Emery's Medical Genetics and Genomics, it "may explain the worldwide distribution of sickle cell disease and malaria."

Summary

Homozygotes (HbSS) are not protected because:

All their RBCs are HbS - sickling is generalized, not selective for parasite-infected cells
The protective mechanism requires a contrast between normal HbA cells and infected/sickling HbS cells - there is no contrast in HbSS
HbSS disease itself kills - chronic hemolytic anemia, vaso-occlusion, and functional asplenia cause early death, negating any fitness benefit
Functional asplenia from repeated splenic infarcts actually leaves HbSS patients MORE susceptible to infections over time

The protection is a privilege of the heterozygous state only - a classic example of balanced polymorphism in human genetics.

Robbins & Cotran Pathologic Basis of Disease, Sickle Cell Disease section
Basic Medical Biochemistry (Lieberman & Peet), Tissue-Specific Isoforms chapter
Lippincott Illustrated Reviews: Biochemistry, Hemoglobin S Disease section

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