Sickle cell disease (SCD) is an autosomal recessive hemoglobinopathy caused by a single nucleotide substitution (GAG→GTG) in the beta-globin gene on chromosome 11, resulting in hemoglobin S (HbS) where valine replaces glutamic acid at position 6. Under conditions of hypoxia, acidosis, dehydration, or temperature extremes, HbS molecules polymerize into rigid rod-like structures that distort the erythrocyte into a sickle (crescent) shape. Sickled cells are rigid and adhesive, causing vaso-occlusion in the microcirculation. Vaso-occlusion triggers an ischemia-reperfusion cascade with endothelial activation, free hemoglobin-mediated nitric oxide scavenging (causing vasoconstriction), inflammatory cytokine release, and platelet activation. This creates a vicious cycle of ongoing sickling and tissue ischemia. Chronic hemolysis (sickled cells have a lifespan of 10-20 days vs normal 120 days) causes anemia, elevated bilirubin (jaundice and cholelithiasis), elevated LDH, and reticulocytosis. The spleen undergoes autoinfarction from repeated vaso-occlusion by age 5 (functional asplenia), making patients highly susceptible to encapsulated organisms (S. pneumoniae, H. influenzae, N. meningitidis).