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Target cells are erythryoctes with an increased cell membrane-to-volume ratio, due either to gain of membrane lipids or to a reduction in cell volume. The characteristic bull’s-eye appearance results from clustering of intensely-staining hemoglobin around the periphery and in the center of the cell. Of note, the cells appear bell-shaped (hence their alternative name, “codocytes” – “bell-cells”) in vivo and on electron microscopy.
Etiologies include thalassemias; hemoglobinopathies C, D, and E; obstructive jaundice; asplenia; and hereditary deficiency of lecithin-cholesterol acyl transferase (LCAT). Target cells may also occur as an artifact, as when smears are dried too rapidly.