Target Cells: Definition and Clinical Significance
Target cells are abnormal red blood cells that appear with a central area of hemoglobin surrounded by a pale zone and an outer ring of hemoglobin, resembling a shooting target on peripheral blood smear.
Morphology and Appearance
Target cells (also called codocytes or leptocytes) have a distinctive appearance:
- On stained blood smears, they show a bull's-eye or target-like pattern with three zones of color
- In three dimensions, these cells exist as thin cup-shaped structures rather than the normal biconcave disc 1
- The characteristic appearance results from an increased surface area-to-volume ratio of the red blood cell membrane
Pathophysiology
The formation of target cells involves specific membrane alterations:
In liver disease and lipid disorders, target cell formation occurs through:
- Increased cholesterol and phospholipid content in red cell membranes 2
- Elevated cholesterol/phospholipid and cholesterol/lecithin mole ratios 2
- These lipid changes correlate with the free cholesterol saturation of serum lipoproteins 2
In hemoglobinopathies (particularly sickle cell disease), the mechanism differs:
- Target cell formation involves a band 3-based mechanism of erythrocyte shape control 1
- The process is driven by slow outward transport of dibasic phosphate with H+ by band 3 protein 1
- This is promoted by decreased Donnan ratio, which decreases cell pH and volume 1
- The volume decrease results from higher K+Cl- cotransport efflux and Ca2+-activated Gardos channel activity 1
Clinical Conditions Associated with Target Cells
Target cells appear in multiple disease states:
Hemoglobinopathies:
Liver disease:
- Stable cirrhosis (particularly alcoholic cirrhosis) 2
- Obstructive jaundice
- Cholestatic liver disease
Other conditions:
- Iron deficiency anemia
- Post-splenectomy states
- Hereditary disorders of lipid metabolism 1
Clinical Distinction: Target Cells vs. Spur Cells
An important clinical caveat: In patients with advanced cirrhosis, target cells and spur cells represent distinct syndromes with different prognoses 2:
- Target cells in cirrhosis are associated with stable disease
- Spur cells (acanthocytes) indicate severe hemolytic anemia and worse prognosis
- These can be distinguished by membrane lipid analysis: spur cells have higher cholesterol/phospholipid ratios but are NOT rich in lecithin, unlike target cells 2
Note: The provided evidence primarily discusses drug targets in biomedical research 3, viral target cells 4, 5, and other unrelated topics 6, 7, 8, 9. The answer above draws from the relevant hematology studies 2, 1 and general medical knowledge to provide a comprehensive explanation of target cells as a hematologic finding.