Leukocytes, more commonly called white blood cells, are cells involved in the immune system. Their name comes from the Greek words leukos meaning white and kutos meaning cell. Given their essential role in the defense of the organism, these cells are particularly monitored during a blood test.

Location

Like red blood cells (red blood cells) and thrombocytes (platelets), white blood cells can circulate in the blood. White blood cells also have the particularity of being present in the lymphatic system (lymph), in some connective tissue and in some lymphoid organs like lymph nodes or spleen.

Synthesis

Leukocytes are cells synthesized at the level of the bone marrow.

Classification

Leukocytes are in fact a large family of cells. Depending on their morphology, these can be classified into three distinct groups:

• Granulocytes, representing between 40% and 80% of the leukocytes present in the body, which group together neutrophils, basophils and eosinophils;
• Lymphocytes, representing between 20% and 40% of total leukocytes, which include B lymphocytes, T lymphocytes and Natural Killer cells (NK cells);
• Monocytes, representing between 2% and 10% of the body’s leukocytes, which include macrophages and dendritic cells.

Leukocytes are part of the body’s defense systems. These cells are involved in the innate immune response and the adaptive immune response.

Innate immune response

This constitutes the first response of the organism during an attack by pathogenic agents. It appears immediately and results in various processes, one of which is inflammatory response and installation of the add-in system. These mechanisms play an essential role inelimination of pathogens. This innate immune system involves several white blood cells including:

• macrophages, which have a phagocyte role, that is, they are able to ingest and destroy pathogens;
• NK cells, which are involved in the destruction of damaged cells such as infected or tumor cells;
• eosinophils, which fight against parasitic infections;
• basophils, which participate in the inflammatory response;
• dendritic cells, which allow the activation of the adaptive immune response.

Adaptive immune response

This follows the innate immune response. Unlike innate immunity, adaptive immunity is said to be non-immediate and specific. This immune response is in fact said to be adaptive because it is based on a pathogen recognition. This involves memorizing these agents for a targeted attack in the event of a new attack. This defense system particularly involves the two types of lymphocytes:

• T lymphocytes, which maintain the immune response and destroy pathogens;
• B lymphocytes, which produce antibodies, complex proteins capable of recognizing and neutralizing pathogens in a targeted manner.

Autoimmune disease

This type of disease is caused by a dysfunction of the immune system. Antibodies produced by B lymphocytes attack cells in the body. There are different autoimmune diseases such as rheumatoid arthritis, multiple sclerosis or type 1 diabetes.

Allergies

Allergic responses are caused by the release of histamine from basophilic granulocytes.

Human Immunodeficiency Virus (HIV)

Infection with this virus causes Acquired Immune Deficiency Syndrome (AIDS). This is characterized by a general weakening of the immune system, which exposes the body to opportunistic infections.

Cancer

Certain cancers can affect white blood cells such as:

• a leukemia, which corresponds to cancer of the cells of the bone marrow;
• un lymphome, which designates cancer of the lymphatic system;
• a myeloma, which is hematologic cancer;
• Waldenström’s disease, which is also hematologic cancer.

HIV prevention

Some abnormalities of the white blood cells may be limited. In particular, HIV infection can be prevented by providing adequate protection during sexual intercourse.

Medical treatments

Depending on the pathology diagnosed, specific medical treatment may be implemented. Antihistamines can be used to fight allergies. During an HIV infection, antiretroviral-based treatments are put in place. During development, chemotherapy or radiotherapy may be used.

Surgical intervention

In the event of severe abnormalities, a bone marrow transplant may be performed. This can especially be the case with leukemia.

Hemograms

This examination makes it possible to carry out a qualitative and quantitative measurement items in the blood including white blood cells and red blood cells. A blood count is used to define leukocytosis, that is to say the blood level of leukocytes. The interpretation of the results of a blood test makes it possible to identify:

• Low leukocytes: Below 3 leukocytes per mm500 (or mL), we speak of leukopenia with an abnormally low level.
• Elevated leukocytes: Above 10 leukocytes per mm000 (or mL), we speak of hyperleukocytosis with an abnormally high rate.

Myelogram

This test measures the production of leukocytes through the study of the bone marrow.

Urine cytobacteriological examination (ECBU)

La presence of leukocytes in the urine can also be evaluated. A high level of leukocytes in the urine is often a sign of an infection in the body. We are talking about leucocyturie.

During a blood test, centrifugation may be performed. This technique reveals a white layer which corresponds to the leukocytes. It is because of this white color that leukocytes are often called white blood cells.