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In contrast to red blood cells, white blood cells (leukocytes) have a nucleus and are independently mobile. Due to their special functions, they are highly differentiated and do not undergo cell division (mitosis) in the bloodstream, although some retain the ability to undergo mitosis. As a group, they are involved in the body’s defense mechanisms and repair activities. The number of white blood cells in normal blood is between 4,500 and 11,000 per cubic millimeter. There are fluctuations throughout the day; lower values ​​are achieved in the resting state and higher values ​​in the stress state. During intense physical exertion the number can exceed 20,000 per cubic millimeter. Most white blood cells are outside the circulation and the few in the bloodstream are on their way from one place to another. As living cells, their survival depends on continuous energy production. The chemical processes used are more complex than those of red blood cells and are similar to those of other tissue cells. White blood cells, which contain a nucleus and are capable of producing ribonucleic acid (RNA), can synthesize proteins. They are made up of three classes of cells, each unique in structure and function, called granulocytes, monocytes and lymphocytes.

What Is The Purpose Of White Blood Cells

Granulocytes, the most numerous white blood cells, are larger than red blood cells (about 12–15 μm in diameter). They have a multilobed nucleus and contain a large number of cytoplasmic granules (i.e. granules in the cell substance outside the nucleus). Granulocytes are important mediators of the inflammatory response. There are three types of granulocytes: neutrophils, eosinophils and basophils. Each type of granulocyte is identified by the color of the granules when the cells are stained with a compound dye. The neutrophil granules are pink, the eosinophil granules are red, and the basophil granules are blue-black. About 50 to 80 percent of white blood cells are neutrophils, while eosinophils and basophils together make up no more than 3 percent.

Basophils: Function & Ranges

The neutrophils are fairly uniform in size, measuring between 12 and 15 μm in diameter. The nucleus consists of two to five lobes connected by hair-like filaments. Neutrophils move with an amoeboid motion. They extend over long projections called pseudopodium into which their granules flow. This process is followed by a contraction of the filaments located in the cytoplasm, which pulls the nucleus and the back of the cell forward. In this way, neutrophils move quickly along a surface. The bone marrow of a normal adult produces about 100 billion neutrophils every day. It takes about a week to form a mature neutrophil from a progenitor cell in the bone marrow; But once in the blood, the mature cells only live for a few hours or perhaps a little longer after migrating into the tissue. To prevent rapid depletion of short-lived neutrophils (e.g. during infection), the bone marrow holds large numbers of them in reserve to be mobilized in response to inflammation or infection. Within the body, neutrophils migrate to areas of infection or tissue injury. The attractive force that determines the direction in which neutrophils move is called chemotaxis and is attributed to substances released at sites of tissue damage. Of the 100 billion neutrophils circulating outside the bone marrow, half are in the tissue and the other half are in the blood vessels. Of those in the blood vessels, half are in the main stream of rapidly circulating blood, and the other half move slowly along the inner walls of the blood vessels (marginal basins) and are ready to penetrate into the tissues when receiving a chemotactic signal from them.

Neutrophils are actively phagocytic; They devour bacteria and other microorganisms as well as microscopic particles. Neutrophil granules are microscopic packets of potent enzymes capable of digesting many types of cellular material. When a bacterium is enveloped by a neutrophil, it becomes surrounded by a vacuole lined by the invaginating membrane. The granules release their contents into the vacuole where the organism is located. This leads to the exhaustion of the neutrophil granules (degranulation). A metabolic process within the granules produces hydrogen peroxide and a highly active form of oxygen (superoxide), which destroy the ingested bacteria. The final digestion of the invading organism occurs through enzymes.

Eosinophils, like other granulocytes, are produced in the bone marrow until they are released into the circulation. Although the eosinophil is approximately the same size as neutrophils, it contains larger granules, and the chromatin is generally concentrated in only two non-segmented lobes. Eosinophils leave the circulation within a few hours of their release from the bone marrow and travel via the lymphatic channels to the tissues (usually those of the skin, lungs and respiratory tract). Like neutrophils, eosinophils respond to chemotactic signals released at the site of cell destruction. They are actively motile and phagocytic. Eosinophils are involved in the defense against parasites and are involved in hypersensitivity and inflammatory reactions, primarily by dampening their destructive effects.

Basophils are the most numerous granulocytes and their large granules almost completely obscure the underlying bilobed cell nucleus. Within hours of their release from the bone marrow, basophils migrate from the circulation to the barrier tissues (e.g., skin and mucosa), where they synthesize and store histamine, a natural modulator of the inflammatory response. When an exacerbation occurs, basophils, along with histamine and other substances, release leukotrienes, which cause bronchoconstriction in anaphylaxis (a hypersensitivity reaction). Basophils, in conjunction with platelets, macrophages and neutrophils, trigger immediate hypersensitivity reactions.

White Blood Cells Use Molecular Breaststroke To Swim

Monocytes are the largest cells in the blood (average 15–18 μm in diameter) and make up about 7 percent of leukocytes. The nucleus is relatively large and is indented or folded rather than multi-lobed. The cytoplasm contains a large number of fine granules, which often appear to be more numerous near the cell membrane. Monocytes are actively motile and phagocytic. They are capable of absorbing infectious agents as well as red blood cells and other large particles, but cannot replace the function of neutrophils in removing and destroying bacteria. Monocytes usually penetrate into inflamed tissue areas later than granulocytes. They are often found in sites of chronic infection.

In the bone marrow, granulocytes and monocytes arise from a common precursor under the influence of granulocyte-macrophage colony-stimulating factor. Monocytes leave the bone marrow and circulate in the blood. After a few hours, the monocytes invade the tissue and develop into macrophages, the tissue phagocytes, which form the reticuloendothelial system (or macrophage system). Macrophages are found in almost all tissues of the body. The ones in the liver are called Kupffer cells and those in the skin are called Langerhans cells. In addition to their role as scavengers, macrophages play a key role in immunity by absorbing and processing antigens so that they can be recognized as foreign substances by lymphocytes.

Lymphocytes make up around 28-42 percent of white blood cells and are part of the immune response to foreign substances. Most lymphocytes are small, only slightly larger than erythrocytes, and their nucleus takes up most of the cell. Some are larger and have more abundant cytoplasm containing some granules. Lymphocytes are sluggishly mobile and their migration routes outside the bloodstream differ from those of granulocytes and monocytes. Lymphocytes are found in large numbers in the lymph nodes, spleen, thymus, tonsils, and lymphoid tissue of the gastrointestinal tract. They enter the circulation via lymphatic channels, which mainly flow into the thoracic lymph duct, which has a connection to the venous system. Unlike other blood cells, some lymphocytes can leave the circulation and re-enter the bloodstream and survive there for about a year or longer. The main route of recirculating lymphocytes is through the spleen or lymph nodes. Lymphocytes freely leave the blood and enter the lymphatic tissue, overcoming barriers that prevent the passage of other blood cells. When stimulated by antigens and certain other agents, some lymphocytes are activated and are capable of cell division (mitosis).

The lymphocytes regulate or are involved in the acquired immunity against foreign cells and antigens. They are responsible for immunological responses to invading organisms, foreign cells such as those of a transplanted organ, and foreign proteins and other antigens that do not necessarily come from living cells. The two classes of lymphocytes are not distinguished by the usual microscopic examination, but rather by the type of immune response they produce. The B lymphocytes (or B cells) are involved in what is known as humoral immunity. When encountering a foreign substance (or antigen), the B lymphocyte differentiates into a plasma cell that secretes immunoglobulin (antibodies). The second class of lymphocytes, the T lymphocytes (or T cells), are involved in regulating the antibody production function of B lymphocytes as well as in directly attacking foreign antigens. T lymphocytes are involved in the so-called cell-mediated immune response. T lymphocytes are also involved in the rejection of transplanted tissue and in certain types of allergic reactions.

Blood: Function, What It Is & Why We Need It

All lymphocytes begin their development in the bone marrow. The B lymphocytes partially mature in the bone marrow until they are released into the bloodstream. The further differentiation of B lymphocytes occurs primarily in lymphatic tissues (spleen or lymph nodes).

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