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CD21 Cell Count (from Whole Blood)

Published: August 9, 2026Updated: August 15, 2026

This test measures the number of CD21-positive cells in your blood, which are part of the immune system. CD21 cells help regulate immune responses and are often evaluated in conditions affecting the immune system.

Quick summary

["CD21 cells play a role in immune function; low levels may indicate immune deficiencies or certain diseases."]

Medically reviewed by WellAlly Editorial TeamLast reviewed August 15, 2026Editorial policy

Key facts at a glance

LOINC
89355-2
Short code
CD21-B
Synonyms
CD21- cells/Cells.CD19 NFr Bld
Category
chemistry
Sample
whole-blood
Container
EDTA (lavender/purple-top) vacutainer tube; ACD (yellow-top) tubes are also acceptable for flow cytometry analysis
Units
N/A
Fasting
false
Population
adult
Sex
all
Age range
≥ 0 years
Method
Flow cytometry using fluorescently labeled monoclonal antibodies against CD19 and CD21 surface markers, with analysis performed on whole blood leukocytes
CPT
86356
SNOMED
395145001
Specimen volume
3-5 mL of whole blood for adults; 1-3 mL for pediatric patients
Stability
Specimen should be processed within 24-48 hours of collection when stored at room temperature (18-25°C). Do not refrigerate or freeze as this may affect cell viability and marker expression. For delayed processing beyond 24 hours, consult laboratory-specific guidelines.
Hemolysis max
50
Lipemia max
100
Icterus max
50
TAT
24–72 hours
Order
routine

Why This Test Might Be Ordered

Your healthcare provider may order this test if you have symptoms or conditions linked to immune system issues, such as chronic infections, autoimmune diseases, or certain blood disorders. It could also be part of a broader evaluation for immune-related problems or before certain treatments that affect the immune system.

What to Expect Before the Test

No special preparation is typically needed. You may have a small needle inserted into a vein in your arm to draw a small amount of blood. If you’re nervous, let your provider know—they can help make the process more comfortable.

What the Test Results Mean

Results are usually reported as a count of CD21 cells per microliter of blood. While specific reference ranges vary by lab, lower than expected counts may suggest immune dysfunction, while higher counts might raise questions about underlying conditions.

Common Conditions Linked to CD21 Levels

Abnormal CD21 levels can be associated with conditions like Wiskott-Aldrich syndrome, common variable immunodeficiency, or other immune deficiencies. It may also be checked in patients with unexplained infections, autoimmune diseases, or before bone marrow transplants.

Reference range

Reference ranges vary by laboratory and population. CD21-negative B cells (CD21-/CD19+) are typically reported as a percentage of total CD19+ B lymphocytes. Generally, CD21- B cells comprise a small subset of CD19+ B cells in healthy individuals. Age-adjusted reference ranges are essential as CD21 expression patterns differ between pediatric and adult populations.N/A

Specimen: whole-blood

No special preparation required. Patients should inform their healthcare provider of any recent vaccinations, infections, or immunosuppressive therapies as these may transiently alter B cell subsets. Avoid collecting samples during acute illness when possible, as immune activation can affect lymphocyte subset distributions.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

◆Associated findings1

Short name

CD21 Count

Frequently asked questions

What does a low CD21 count mean?
A low CD21 count may indicate an immune system that isn’t functioning as expected, potentially linked to inherited or acquired immune deficiencies. Your provider will discuss this with you based on your symptoms and medical history.
What does a high CD21 count mean?
High CD21 levels are less common but could suggest conditions like certain lymphoproliferative disorders or immune activation. Further testing and evaluation by your healthcare team will help clarify the cause.
Can diet or lifestyle affect CD21 test results?
While diet or lifestyle factors may influence overall immune health, they typically do not directly affect CD21 cell counts in blood tests. Your provider will focus on medical reasons for any changes in results.
  1. 1
    MedlinePlus — Laboratory TestsSource
  2. 2
    Lab Tests OnlineSource
  3. 3
    Mayo Clinic — Tests & ProceduresSource

Click any link to access the full source. PubMed Central (PMC) links provide free full-text access.

This information is for educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment.

Medical Disclaimer

This content is for general informational and educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always seek the guidance of a qualified healthcare provider with any questions you have about a medical condition, and never disregard professional medical advice or delay seeking it because of something you read here.

If you think you may have a medical emergency, call your local emergency number or go to the nearest emergency department immediately.

Medically reviewed by WellAlly Editorial Team Last reviewed August 15, 2026. · Read our editorial policy