target cell

**Morphology:** On peripheral smear, the target cell (codocyte) shows a central hemoglobinized area, a surrounding pallor ring, and a peripheral rim of hemoglobin, giving a "bullseye" look. The cell is thin and bell-shaped in three dimensions, which is why it looks like a target when flattened on a slide.

**Pathophysiology:** Target cells form when the RBC membrane surface area is excessive relative to the cell volume (or hemoglobin content). Two main mechanisms: 1. Reduced hemoglobin or cytoplasmic volume with normal membrane. This occurs in thalassemia (decreased globin chain synthesis) and iron deficiency anemia (decreased heme). 2. Increased membrane surface area with normal volume. This occurs in liver disease and obstructive jaundice, where excess cholesterol and phospholipid (lecithin) accumulate in the membrane because of LCAT deficiency or bile-salt abnormalities. It also occurs in asplenia or hyposplenism, because the spleen no longer remodels and removes these cells. 3. Hemoglobin C disease (HbC crystals, glutamate to lysine at β6) causes a characteristic dehydrated, target-cell-rich smear.

**Mnemonic: HALT** - **H**emoglobin C disease (also HbS/HbC, HbSC) - **A**splenia (post-splenectomy, functional asplenia in sickle cell disease) - **L**iver disease (cholestasis) - **T**halassemia (also iron deficiency)

**Clinical pearls:** - Thalassemia: microcytic anemia with a normal or high RBC count, a low Mentzer index (MCV/RBC < 13), target cells, basophilic stippling, and a normal or high ferritin. HbA2 is raised in β-thalassemia trait. - Iron deficiency: microcytic anemia with target cells, pencil cells, and anisopoikilocytosis, plus low ferritin, high TIBC, and high RDW. - Hemoglobin C disease: mild hemolytic anemia with many target cells and sometimes intracellular HbC crystals. It is often asymptomatic. - Post-splenectomy smear: target cells, Howell-Jolly bodies, Heinz bodies, and acanthocytes. - Target cells help distinguish some microcytic anemias from anemia of chronic disease, which usually lacks them.

**Related smear findings (for contrast):** Spherocytes (hereditary spherocytosis and warm AIHA) have no central pallor, schistocytes (MAHA, DIC, TTP/HUS) are fragmented cells, and teardrop cells (dacrocytes) occur in myelofibrosis.