When the number of lymphocytes, one of the white blood cells in the blood, is over 4500/mm3, it is called lymphocytosis. When it rises above 9000/mm3 in children, lymphocytosis is mentioned.


When the number of lymphocytes, one of the white blood cells in the blood, is over 4500/mm3, it is called lymphocytosis. When it rises above 9000/mm3 in children, lymphocytosis is mentioned.

Lymphocytes constitute about half of the circulating leukocytes in the blood and there are many subtypes. It is usually not known which subtype lymphocyte group is increased in the complete blood count.

Lymphocytes are the most important elements of the immune system. Their most interesting feature is that they can divide themselves and multiply by forming new lymphocytes. Lymphocytes, which are in a calm state in the blood circulation, can immediately differentiate and switch to the form of defense cells when they receive any foreign stimulus. When inactive, they look alike. However, when they differentiate, the sensor receptors and molecules on their surfaces also differentiate. Lymphocytes are also stored in connective tissues other than blood vessels. Lymphocytes in the blood can go to the tissues when necessary and return to the blood when their task is over. In addition, there are lymphocyte localizations outside the vessels in some organs.

Specific groups of lymphocytes show special reactions for specific reasons. In order to make these distinctions, lymphocytes have memories.

Lymphocytes are divided into three according to their shape:

  • Small lymphocytes with narrow cytoplasm
  • Large lymphocytes with extensive cytoplasm
  • Large lymphocytes with granules in the cytoplasm

In terms of their functions, lymphocytes are divided into different groups:

  • B lymphocytes
  • T lymphocytes
  • NK cells
  • And their subgroups

It is not possible to recognize all types of lymphocytes under the microscope. Immunophenotyping is required.

When an increase in lymphocyte count is detected with a complete blood count (hemogram), it is investigated whether this is a real situation. Because sometimes there can be relative heights. For example, the decrease in blood volume may be reflected as a relative increase in the number of lymphocytes per unit volume. In this case, an increase in other blood cells can also be seen.

Which group of lymphocytes increases is important for diagnosis. While T-cell lymphocytes increase in leukemia patients, it is usually group B lymphocytes in reactive lymphocyte increases that develop in response to any situation.

Lymphocytes often increase reactively as the body’s defense response to infections. This is more pronounced in children and young adults. An increase in lymphocytes in advanced ages brings to mind leukemias.

The most common causes of lymphocytosis are:

  • Primary lymphocytosis
    • Lymphocytic Cancers
      • Acute and Chronic Lymphocytic Leukemia
      • Hairy Cell Leukemia
      • Adult T Cell Leukemia
      • Large Granular Lymphocytic Leukemia
    • Essential Monoclonal B-Cell Lymphocytosis
    • Persistent Polyclonal B-Cell Lymphocytosis
  • Reactive lymphocytosis
    • Infectious Causes
      • Ebstein Barr Virus
      • Cytomegalovirus
      • HIV
      • Human Herpes virus (types 6 and 8)
      • Herpes Simplex virus type II
      • Toxoplasma Gondii
      • Bordetella Pertussis
      • Rubella virus
      • Infectious Hepatitis virus
      • Adenovirus
      • Varicella Zoster Virus
      • Brucella
    • Stress Lymphocytosis (Acute)
      • Cardiovascular Collapse (Acute Heart Failure, M. Infarction)
      • Staphylococcal Toxic Shock Syndrome
      • Major Surgery, trauma
      • Status Epilepticus
    • Hypersensitivity Reaction
      • Medicines
        • Para-aminosalicylic acid
        • Diphenylhydantoin
        • Mephenitonin
      • Insect bite
      • Blood transfusion reaction
    • Persistent Lymphocytosis (Subacute or chronic)
      • Cigarette
      • Cancer
      • Hyposplenism

When lymphocyte elevation is detected in the hemogram, the change in the patient’s blood volume and whether there is a relative increase are examined. If true lymphocyte increase is detected by peripheral smear, it is investigated whether the lymphocytes are increased due to any infection or a leukemia. Lymphocytosis alone does not usually cause clinical problems in the patient.


Lymphocytes, one of the white blood cells in the blood, are expected to be between 1500-4000/mm3 in normal situations. This rate can be up to 10 thousand in children and 17 thousand in infants. Lymphocyte count below 1500/mm3 is called lymphopenia.

Causes of decrease in lymphocyte count:

  • Decreased lymphocyte production
    • protein calorie malnutrition
    • Radiation
    • Immunosuppressive agents (steroids, cyclosporines, fludarabine, cladribine)
    • Congenital immunodeficiencies
      • Wiskott-aldrich syndrome
      • Nezelof syndrome
      • Adenosine deaminase deficiency
    • Viral infections (HIV, SARS, SARS-COV2, influenza, hepatitis)
    • Bacterial infections (Tuberculosis, Brucellosis)
    • Fungal infections (Histoplasmosis)
    • Parasitic diseases (Malaria/Malaria)
    • Hodgkin’s disease
    • Multiple myeloma
    • Diffuse granulomatous disease
    • Cytotoxic chemotherapy
    • Drug reactions (Alemtuzumab, rituximab, quinine etc.)
  • Disturbances in lymphocyte traffic
    • Acute bacterial fungal infections
    • Surgical
    • Trauma
    • Bleeding
    • Glucocorticoids
    • Viral infection
    • Disseminated granulomatous infection
    • Hodgkin lymphoma
  • It may occur due to loss or excessive destruction of lymphocytes.
    • Viral infections
    • Antibody-mediated lymphocyte destruction
    • Protein-losing enteropathies
    • Chronic right ventricular failure
    • Duct thoracic drainage and rupture
    • Extracorporeal circulation (dialysis)
    • Graft versus host reaction

The most common causes of lymphopenia are:

  • Acute infections
  • Tuberculosis
  • Cushing’s syndrome and steroid use
  • Chemotherapeutic agents
  • Radiation
  • Some cancers
  • Immunodeficiencies (AIDS)
  • Systemic Lupus Erythematosus
  • Rheumatoid arthritis
  • Sjogren’s syndrome
  • Kidney failure