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'''Leukocytes''', also called '''white blod cells''', 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.
'''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.


There are multiple subgroups of leukocytes, including granulocytes, lymphocytes, and monocytes. We distinguish three types of granulocytes, neutrophils, eosinophils, and basophils. Granulocytes received their name due to the appearance of granules in their cytoplasm.  
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.


== Neutrophils ==
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:
{| class="wikitable"
|'''Cell type'''
|'''Number'''
|-
|White blood cell (WBC)
|4 000 – 11 000/µL
|-
|'''N'''eutrophils
|60% of all WBCs
|-
|'''L'''ymphocytes
|35% of all WBCs
|-
|'''M'''onocytes
|5% of all WBCs
|-
|'''E'''osinophils
|3% of all WBCs
|-
|'''B'''asophils
|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.
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.


Levels of neutrophils in the blood usually increases in response to bacterial infections.
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 ([[Phagocytosis|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 cells|NK (Natural Killer) cells]].
 
<abbr>NK cells</abbr> 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 [[Glucocorticoid|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.


Their biological function is [[phagocytosis]] and [[chemotaxis]].
The type of leukocyte which is elevated or decreased can be valuable in determining the cause:
{| class="wikitable"
|+
!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]]
|}
[[Category:Immunology]]
[[Category:Immunology]]
[[Category:Laboratory Medicine]]
[[Category:Laboratory Medicine]]
[[Category:Physiology]]