Lymphocytes are white blood cells (leukocytes) whose role is the immune defense of the body against infectious aggressions.
The main ones are B lymphocytes and T lymphocytes.
In case of a decrease (lymphopenia) or increase (lymphocytosis), you should consult a doctor.
What are lymphocytes?
Lymphocytes represent between 20% and 40% of circulating white blood cells.
They are small and originate from the lymphoid organs, which are themselves separated into two groups:
- the primary lymphoid organs, which are the thymus and the bone marrow;
- the secondary lymphoid organs, which are the spleen and the lymph nodes. There are three groups of lymphocytes.
The two main groups are B and T lymphocytes, and the third group is represented by NK lymphocytes. The role of lymphocytes in the defense against infectious aggression is preponderant.
T lymphocyte
T lymphocytes (for “Thymus” because they finish their maturation in the thymus), are responsible for the so-called “cellular” immunity by destroying the cells recognized as infected. They represent 80% of lymphocytes.
In the presence of a microbe, the T lymphocytes multiply and associate with other types of white blood cells to get rid of the cellular threat.
When the microbe has reached the nucleus of the cell, the T cells destroy the cell. They can fight bacteria, viruses or fungi.
B lymphocyte
B lymphocytes produce immunoglobulins, proteins whose role as antibodies is to destroy molecules recognized as foreign to the body.
They represent 10% of lymphocytes. In case of attack by a pathogen, B lymphocytes complete their maturation and multiply to bring an adapted immune response. These B lymphocytes become plasma cells, cells that secrete antibodies.
What is absolute lymphocyte count (alc)?
An absolute lymphocyte count (ALC) represents the number of lymphocytes cells in your white blood cell. This count is expressed in absolute count instead of a percentage.
How Is Absolute Lymphocyte Count (ALC) Calculated?
The absolute lymphocyte count (ALC) is calculated using the total white blood cell (WBC) count and the percentage of lymphocytes reported on a complete blood count (CBC) with differential.
The formula is:
ALC = Total WBC count × Percentage of lymphocytes
For example:
If your WBC count is 8,000 cells/mm³ and lymphocytes represent 50%, the calculation would be:
8,000 × 0.50 = 4,000 lymphocytes/mm³
This value represents your true lymphocyte level in circulation.
It is important to understand that physicians rely more heavily on the absolute lymphocyte count than on the percentage alone. A high percentage does not always indicate a true elevation. For instance, if the total white blood cell count is low, the percentage of lymphocytes may appear elevated even when the absolute count is within normal limits. For this reason, interpretation should always focus on the ALC rather than the percentage in isolation.
What are the functions of lymphocytes?
Lymphocytes are white blood cells that play an important role in the immune system. Within the body, each type of lymphocyte has a specific function in fighting pathogens.
Role of NK cells in the innate immune response
NK lymphocytes, or NK cells, are involved in the innate immune response, which is the body’s first response to an attack by pathogens. The innate immune response is immediate and involves NK cells, whose role is to destroy altered cells such as infected cells and cancer cells.
Roles of B and T cells in the adaptive immune response
B and T Lymphocytes are involved in the adaptive immune response. Unlike the innate immune response, this second phase of the immune response is called specific. Based on the recognition and memorization of pathogens, the adaptive immune response involves several leukocytes including :
- B lymphocytes which produce antibodies, complex proteins with the capacity to recognize and neutralize pathogens in a specific way;
- T lymphocytes that recognize and destroy pathogens in a specific way.
Lymphocytes normal range
Usually, lymphocytes count is analyzed through a complete blood count (CBC).
The normal lymphocyte count should be between 1500 and 4000/mm3 or between 20% and 40% of the total white blood cell count.
In children, it can normally be as high as 7000/mm3.
The standards vary according to the laboratory. The lymphocyte count varies rapidly within the same individual and is always higher in chronic smokers.
Lymphocyte Count Severity Levels: When Is It Mild, Moderate, or Severe?
Understanding the degree of abnormality helps determine how concerning a lymphocyte result may be.
Lymphopenia (Low Lymphocytes)
- Mild: 1,000–1,500/mm³
- Moderate: 500–1,000/mm³
- Severe: Below 500/mm³
Severe lymphopenia significantly increases the risk of infections, particularly viral and opportunistic infections. When levels fall below 500/mm³, medical evaluation is typically required to determine the underlying cause.
Lymphocytosis (High Lymphocytes)
- Mild: 4,000–8,000/mm³
- Moderate: 8,000–15,000/mm³
- Marked elevation: Above 15,000–20,000/mm³
Mild elevations are commonly seen in viral infections and are often temporary. However, persistent lymphocyte counts above 10,000/mm³, especially without clear infection, require medical evaluation to rule out hematologic conditions such as chronic lymphocytic leukemia (CLL) or other lymphoproliferative disorders.
Trends over time are more important than a single isolated result.
What causes low lymphocytes?
The lymphocyte count is low when it is less than 1500/mm3. This phenomenon called lymphopenia leads to an immune deficiency and it can be the consequence of :
- Diseases affecting the blood such as leukemia
- Certain cancers also cause a decrease in lymphocytes.
- The administration of drugs such as immunosuppressants or enter in the framework of chemotherapy used against certain cancers.
Cancer treatments, which are sometimes severe, can cause bone marrow aplasia, i.e. the bone marrow no longer produces blood cells.
The subjects, then at very high risk of infectious pathology, are placed in isolation. A fever associated with a low lymphocyte count should be an urgent warning.
What causes high lymphocytes?
High lymphocytes also called Hyperlymphocytosis or lymphocytosis, corresponds to a lymphocyte count of more than 8,000 to 9,000 /mm3. Elevated lymphocytes are the sign of an infectious disease, most often viral. It can be an ENT infection (mononucleosis, angina), bronchitis or any other infectious disease.
Lymphocytes can also be elevated in cases of cancer or lymphoma.
They are also often higher in smokers.
What are the symptoms of high and low lymphocytes?
High lymphocytes also called lymphocytosis may be reflected in the presence of lymph nodes (in the neck, armpit or groin) that indicate hematologic disease. We also always look to see if the patient has a large liver or a large spleen, which are nodes in their own right. Tumor disease can also cause fever, night sweats and weight loss.
Low lymphocytes also called lymphopenia can result in a chronic infectious syndrome characterized by fever, cough and runny nose, which can be found in bronchitis, sinusitis, pneumonia, etc. Nodes (in cancer or HIV infection), swollen and painful joints or a rash (in lupus or rheumatoid arthritis) may also occur.
Lymphocyte Development and Maturation
Lymphocytes originate from hematopoietic stem cells in the bone marrow, but follow distinct developmental pathways.
T cells migrate from the bone marrow to the thymus where they undergo a rigorous selection process, eliminating cells that react to self-antigens (negative selection) while preserving those that can recognize foreign threats (positive selection). B cells mature within the bone marrow through similar selection processes. Both cell types then migrate to secondary lymphoid organs like the spleen and lymph nodes, where they await activation.
NK cells develop primarily in the bone marrow, liver, and thymus, maturing through stages marked by the progressive acquisition of specific surface receptors. This carefully orchestrated development ensures the immune system can distinguish between self and non-self, a fundamental principle that prevents autoimmunity while enabling effective defense against pathogens.
Relative vs Absolute Lymphocytosis: What’s the Difference?
One of the most common sources of confusion is the difference between relative lymphocytosis and absolute lymphocytosis.
- Relative lymphocytosis means the percentage of lymphocytes is high compared to other white blood cells.
- Absolute lymphocytosis means the total number of lymphocytes (ALC) is elevated.
For example:
If the WBC count is 3,000/mm³ and lymphocytes represent 50%, the ALC is:
3,000 × 0.50 = 1,500/mm³
Although 50% appears high, the absolute count (1,500) is still within the normal range. In this case, there is no true lymphocytosis.
This distinction is critical in medical interpretation. Physicians always prioritize the absolute lymphocyte count when assessing whether the elevation is clinically meaningful.
When Should You Be Concerned About Your Lymphocyte Count?
Most mild fluctuations in lymphocyte levels are temporary and related to infections or stress. However, medical evaluation is recommended in the following situations:
- ALC below 500/mm³
- Persistent ALC above 10,000/mm³
- Unexplained weight loss
- Night sweats
- Persistent fever
- Enlarged lymph nodes
- Enlarged spleen
- Recurrent or unusual infections
A single abnormal value is often less concerning than a persistent trend. If lymphocyte levels remain abnormal over several weeks or months, further testing — such as repeat CBC, peripheral blood smear, or immunophenotyping — may be required.
Prompt evaluation is especially important if abnormal counts are accompanied by systemic symptoms.
Educational notice: This content is provided for informational and educational purposes only and is not intended as medical advice. Always consult a qualified healthcare professional for medical concerns.
