Leukocyte

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Leukocytes, also called white blod cells (WBCs), are a population of cells in the blood responsible for immune response, inflammation, and microbe defence. These are always present in the blood, with their numbers increasing in response to infection or inflammation. They are a major part of the immune system.

If the number of WBCs is elevated above 11 000/µL there is leukocytosis. The body produces more WBCs in response to most infections and inflammations in the body. An elevated white blood count on a blood test is almost always indicative of an ongoing infection or inflammation in the patient. In theory strenuous exercise and emotional stress can cause leucocytosis too.

If the number of WBCs is below 4 000/µL there is leukopaenia. This occurs if there is damage to the bone marrow, due to toxic effects from drugs, radiation, chemotherapy, or cancer of the bone marrow, such as myelodysplastic syndrome or leukaemia.

Types

There are multiple subgroups of leukocytes, including granulocytes, lymphocytes, and monocytes. Neutrophils, eosinophils and basophils all contain granules and have a granular appearance. For this reason, they’re together called granulocytes. Monocytes and lymphocytes don’t have granular appearance and are sometimes called agranulocytes.

Recall the number and types of WBCs:

Cell type	Number
White blood cell (WBC)	4 000 – 11 000/µL
Neutrophils	60% of all WBCs
Lymphocytes	35% of all WBCs
Monocytes	5% of all WBCs
Eosinophils	3% of all WBCs
Basophils	0-1% of all WBCs

A good way to remember the order of white blood cells in decreasing order from most abundant to least abundant is the pneumonic never let monkeys eat bananas. N – L – M – E – B.

Neutrophils

Neutrophil granulocytes, often called just neutrophils, are the most abundant leukocyte in the blood, accounting for approx 60% of them. They have a segmented nucleus.

In the blood vessels, there are two populations of neutrophils, approximately equal in size. 50% of them are circulating in the blood and the other 50% of them are bound to the vessel walls. When there is an increased need for neutrophils, these bound neutrophils can mobilise and start to circulate in the blood within 30 minutes.

If there is an increase in just neutrophils, it’s called neutrophilia. A decrease is called neutropaenia. Neutrophils are important in bacterial infections, sterile inflammation and in tissue necrosis, which occurs in burns and myocardial infarction (heart attack), for example. Neutrophilia is expected in these cases.

Neutrophils can eat (phagocytose) pathogens like bacteria and viruses. Their granules contain proteases (enzymes which can break up proteins). They're also involved in chemotaxis.

Left shift and right shift

Under physiological conditions there are very few immature neutrophils in the blood. Mature neutrophils have segmented nuclei, so mature neutrophils are synonymous with segmented neutrophils. Immature neutrophils on the other hand don’t have segmented nuclei but rather nuclei in bands. Two “banded” neutrophils exist, the “jugend” form and the “stab” form. The jugend form is the more immature and less abundant of the two.

In some pathological conditions the number of immature neutrophils in the blood increases compared to normal. This is called a left shift and can occur when immature neutrophils are suddenly released from the bone marrow because the body’s demand for WBCs increases. This can occur in the case of infection and fever.

Similarly, a right shift refers to when the number of immature neutrophils in the blood is lower than normal. This occurs when the production of neutrophils in the bone marrow is supressed, such as in radiation exposure.

Lymphocytes

Three types of lymphocytes exist: T cells, B cells and NK (Natural Killer) cells.

NK cells kills tumor cells or virus-infected cells by causing them to lyse.

T cells help to regulate the immune system and they can force tumor cells or virus-infected cells to lyse.

B cells produce immunoglobulins (antibodies).

Lymphocytes are important in viral infections. Lymphocytosis is expected in these cases.

Monocytes

Monocytes like neutrophils can phagocytose viruses and bacteria. They can also migrate into peripheral tissues (like skin, etc) and transform into macrophages.

Monocytes are important in bacterial infections (especially chronic ones like tuberculosis) and systemic autoimmune diseases. Monocytosis is expected in these cases.

Eosinophils

Eosinophils are important in parasitic infections and allergic disorders, like asthma and hay fever. Eosinophilia is expected in these cases.

The granules of eosinophils contain histaminase and heparinase. In other words, the granules of eosinophils can inactivate the content of the granules of basophils and therefore regulate them.

Basophils

Like eosinophils, basophils are important in parasitic infections. Basophilia often occurs with eosinophilia.

The granules of basophils contain histamine and heparin (an anticoagulant).

Leukocytes as a lab test

Measurement of the number of leukocytes in the blood, a leukocyte count, is often used in the laboratory evaluation of infection or inflammation, leukaemia, and bone marrow. One can order the lab to just measure the number of leukocytes, or one can order the lab to measure the number of each type of leukocytes, called a complete blood count (CBC). Doing a CBC gives more information than a plain leukocyte count, but the information may not always be useful.

Interpretation

Leukocytosis is a sign of inflammation. In case of leukaemia, the leukocyte count can increase beyond 50 000/µl. Other causes of leukocytosis include physical activity, stress, and use of glucocorticoids.

Leukocytopaenia is a sign of bone marrow depression, which can be due to toxic effects of medication or bone marrow malignancy. Sepsis may also cause leukocytopaenia due to increased consumption of leukocytes. A neutrophil count of < 500/µl is severe and a significant risk factor for infection.

The type of leukocyte which is elevated or decreased can be valuable in determining the cause:

Finding	Associated with
Neutrophilia	Bacterial infection, fungal infection, leukaemia, glucocorticoids
Neutropaenia	Bone marrow depression, sepsis
Lymphocytosis	Viral infections, pertussis, lymphoid malignancy
Lymphocytopaenia	Bacterial infection, fungal infection, HIV, glucocorticoids
Monocytosis	Chronic infection, myeloproliferative disorder
Monocytopaenia	Glucocorticoids
Eosinophilia	Allergic reaction, parasitic infection
Basophilia	Allergic reaction, chronic myeloid leukaemia