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B Lymphocytes, Whole Blood

Published: August 9, 2026Updated: August 16, 2026

This blood test measures the number or proportion of B lymphocytes, immune cells that help protect the body from infections.

Quick summary

["The result shows how many B cells are present and may help evaluate immune-system function."]

Medically reviewed by WellAlly Editorial TeamLast reviewed August 16, 2026Editorial policy

Key facts at a glance

LOINC
35040-5
Short code
Ib Lymphs # Bld
Synonyms
Ib Lymphs # Bld
Category
liver
Sample
whole-blood
Container
Lavender-top (EDTA)
Units
N/A
Fasting
false
Population
adult
Sex
all
Age range
18–65 years
Specimen volume
3 mL
Stability
Room temperature: 4h, Refrigerated: 24h
Hemolysis max
0
Lipemia max
0
Icterus max
0
TAT
4–24 hours
Order
routine

What is this test?

A B-lymphocyte test measures the number or proportion of B cells in whole blood. B cells are part of the immune system and produce antibodies that help fight infections.

Why might I need this test?

This test may be used to investigate repeated infections, immune-system disorders, autoimmune conditions, or responses to treatments that affect immune cells. It may also be included as part of a broader lymphocyte-subset evaluation.

How is the test performed?

A blood sample is collected, and the laboratory measures the B cells. Depending on the test, this may be reported as a count, a percentage of lymphocytes, or both, and the measurement method may include flow cytometry.

How do I prepare for the test?

No special preparation is usually required. Follow any instructions from your clinician, and tell the laboratory if you recently received a blood transfusion or take medicines that may affect immune-cell levels.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

A low B-cell count or percentage may occur with certain immune deficiencies, some autoimmune diseases, or medicines that suppress the immune system. A higher result may occur with some infections, inflammatory conditions, or other medical situations, so results must be interpreted with symptoms, medical history, and other laboratory findings.

Short name

B-cell count

Frequently asked questions

What does a B-lymphocyte test measure?
It measures the number or proportion of B cells in the blood. These cells make antibodies and are an important part of immune defense.
Is fasting required for this test?
Fasting is generally not required unless your clinician specifically requests it. Confirm any preparation instructions before the blood draw.
Does an abnormal result mean I have an immune disorder?
Not necessarily. B-cell levels can change with infections, medications, and other conditions, so your clinician should interpret the result alongside your symptoms and other test findings.
  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 16, 2026. · Read our editorial policy