15. Characterize the various leukocytes indicating their origins and functions

Last updated on October 6, 2020 at 12:40

White blood cells

White blood cells (WBCs) are the cells which combat infections. They are a major part of the immune system. 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, 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.

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.

Neutrophils

There can also be changes in the number of individual types of WBCs. 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).

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).


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