Table of Contents

What are Target Cells? A Comprehensive Guide

Target cells, also known as codocytes, are abnormally shaped red blood cells (erythrocytes) that resemble a bullseye or target. They are characterized by a central, dark spot surrounded by a pale area, and then a dark outer ring. This distinctive appearance arises due to an increased surface area to volume ratio in the red blood cell membrane. This excess membrane leads to a folding of the cell upon itself, creating the characteristic target shape.

Understanding the Morphology of Target Cells

The unique morphology of target cells is crucial for identifying them under a microscope. The central spot and outer ring are areas of increased hemoglobin concentration, while the paler zone between them is an area of lower hemoglobin concentration. Understanding the underlying mechanisms that cause this abnormal distribution is key to diagnosing the associated conditions.

The Mechanism Behind the “Bullseye”

The increased surface area relative to the cell’s volume is the primary driver of target cell formation. This can occur due to:

Increased Membrane Lipids: Conditions like liver disease can lead to an increase in cholesterol and phospholipids within the red blood cell membrane, expanding its surface area.
Decreased Intracellular Hemoglobin: Conditions like thalassemia and iron deficiency anemia can result in reduced hemoglobin production, leading to a relative excess of membrane.
Splenectomy: The spleen plays a role in “culling” abnormal red blood cells. After splenectomy, the body is less efficient at removing these cells, so their population in circulation increases.

Clinical Significance: When Target Cells Matter

The presence of target cells is not a disease in itself but rather an indicator of an underlying condition. Their identification during a blood smear examination can be a vital clue for diagnosis. Common conditions associated with target cells include:

Liver Disease: Particularly obstructive jaundice and other forms of cholestasis, causes changes in lipid metabolism that affect red blood cell membranes.
Hemoglobinopathies: Conditions like thalassemia and hemoglobin C disease lead to abnormal hemoglobin production, affecting red blood cell morphology.
Iron Deficiency Anemia: Decreased iron leads to reduced hemoglobin synthesis and ultimately leads to target cell formation.
Post-Splenectomy: Absence of the spleen increases the lifespan of abnormal red blood cells, including target cells.
Lecithin-Cholesterol Acyltransferase (LCAT) Deficiency: LCAT is an enzyme responsible for cholesterol metabolism. Its deficiency can lead to abnormal lipid deposition on red blood cells, affecting their shape.
Other Conditions: Target cells can also be seen, although less frequently, in certain autoimmune disorders, severe burns, and some cases of renal disease.

Diagnosis and Evaluation

Identification of target cells typically occurs during a peripheral blood smear review. Laboratory technologists or hematologists will assess the percentage of target cells present and correlate this with other blood counts and clinical information.

Further Testing

Once target cells are identified, further diagnostic testing is usually necessary to determine the underlying cause. This may include:

Complete Blood Count (CBC): To assess red blood cell indices, such as Mean Corpuscular Volume (MCV) and Mean Corpuscular Hemoglobin (MCH).
Iron Studies: To evaluate iron levels, iron-binding capacity, and ferritin levels.
Liver Function Tests (LFTs): To assess liver health.
Hemoglobin Electrophoresis: To identify abnormal hemoglobin variants.
Genetic Testing: May be required for definitive diagnosis of hemoglobinopathies such as thalassemia.

Treatment and Management

Treatment for target cells is directed at the underlying cause of the abnormality. There is no specific treatment to correct the shape of the red blood cells themselves. For example:

Iron Deficiency Anemia: Iron supplementation is the primary treatment.
Liver Disease: Management focuses on addressing the underlying liver condition.
Thalassemia: Depending on the severity, treatment may involve blood transfusions, iron chelation therapy, or bone marrow transplantation.

Frequently Asked Questions (FAQs)

1. Are Target Cells Always a Sign of Disease?

While target cells usually indicate an underlying condition, they can sometimes be found in small numbers in healthy individuals. However, a significant number of target cells warrants further investigation.

2. What is the Normal Range of Target Cells in a Blood Smear?

Ideally, no target cells should be present in a normal blood smear. However, a very small number (less than 5%) may be considered within the realm of normal, particularly after a splenectomy. Anything above this level should be investigated.

3. Can Medications Cause Target Cells?

Certain medications can affect liver function or red blood cell production, potentially contributing to target cell formation. Consult your doctor about your medications if target cells are detected.

4. How are Target Cells Different from Other Abnormal Red Blood Cell Shapes?

Target cells have a very distinct “bullseye” appearance, unlike other abnormal red blood cell shapes like spherocytes (spherical cells), schistocytes (fragmented cells), or sickle cells (crescent-shaped cells).

5. Does the Number of Target Cells Correlate with the Severity of the Underlying Condition?

Generally, a higher percentage of target cells indicates a more significant or long-standing underlying condition. However, this is not always the case, and the clinical context must be considered.

6. Can Target Cells Cause Any Symptoms?

Target cells themselves do not directly cause symptoms. However, the underlying conditions that lead to their formation can cause a variety of symptoms depending on the disease.

7. How Long Does it Take for Target Cells to Disappear After Treating the Underlying Condition?

The time it takes for target cells to decrease or disappear after treatment depends on the underlying condition and the effectiveness of the treatment. It can range from weeks to months.

8. Can Target Cells be Prevented?

Preventing target cells involves preventing or managing the underlying conditions that cause them. For example, iron deficiency anemia can be prevented through adequate iron intake.

9. Is a Splenectomy Always a Cause for Target Cells?

A splenectomy increases the likelihood of finding target cells on a blood smear, but it is not the sole cause. Other conditions can still contribute to their formation.

10. Are Target Cells More Common in Certain Populations?

Yes, target cells are more common in populations with a higher prevalence of hemoglobinopathies, such as thalassemia and sickle cell anemia, or in regions where liver disease is more prevalent.

11. What Happens if the Underlying Cause of Target Cells is Not Treated?

If the underlying cause is left untreated, the condition that is causing the target cells will likely worsen. This can lead to further complications and health problems.

12. Where Can I Find More Information about Target Cells and Related Conditions?

Reliable sources of information include your healthcare provider, reputable medical websites like the Mayo Clinic and the National Institutes of Health (NIH), and hematology textbooks.