WHITE BLOOD CELLS

White blood cells are blood components that protect the body from infectious agents. Also called leukocytes, white blood cells play an important role in the immune system by identifying, destroying, and removing pathogens, damaged cells, cancerous cells, and foreign matter from the body. Leukocytes originate from bone marrow stem cells and circulate in blood and lymph fluid. Leukocytes are able to leave blood vessels to migrate to body tissues. White blood cells are categorized by the apparent presence or absence of granules (sacs containing digestive enzymes or other chemical substances) in their cytoplasm. A white blood cell is considered to be a granulocyte or an agranulocyte.

Granulocytes

There are three types of granulocytes: neutrophils, eosinophils, and basophils. As seen under a microscope, the granules in these white blood cells are apparent when stained.

• Neutrophils - These cells have a single nucleus that appears to have multiple lobes. Neutrophils are the most abundant granulocyte in blood circulation. They are chemically drawn to bacteria and migrate through tissue to the site of infection. Neutrophils are phagocytic in that they engulf the target cell (bacterium, diseased or dead cell, etc.) and destroy it. When released, neutrophil granules act as lysosomes to digest cellular macromolecules. The neutrophil is also destroyed in the process.
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• Eosinophils - The nucleus in these cells is double-lobed and often appears U-shaped in blood smears. Eosinophils are often found in connective tissues of the stomach and intestines. Eosinophils are phagocytic and primarily target antigen-antibody complexes. These complexes are formed when antibodies bind to antigens to identify them as substances to be destroyed. Eosinophils become increasingly active during parasitic infections and allergic reactions.
• Basophils - Basophils are the least numerous of the white blood cells. They have a multi-lobed nucleus, and their granules contain substances such as histamine and heparin. Heparin thins blood and inhibits blood clot formation. Histamine dilates blood vessels, increases the permeability of capillaries, and increases blood flow, which helps to transport leukocytes to infected areas. Basophils are responsible for the body's allergic response.
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  Agranulocytes

  There are two types of agranulocytes, also known as nongranular leukocytes: lymphocytes and monocytes. These white blood cells appear to have no obvious granules. Agranulocytes typically have a large nucleus due to the lack of noticeable cytoplasmic granules.
  
• Lymphocytes - After neutrophils, lymphocytes are the most common type of white blood cell. These cells are spherical in shape with large nuclei and very little cytoplasm. There are three main types of lymphocytes: T cells, B cells, and natural killer cells. T cells and B cells are critical for specific immune responses. Natural killer cells provide nonspecific immunity.
• Monocytes - These cells are the largest of the white blood cells. They have a large, single nucleus that can have various shapes. The nucleus often appears to be kidney-shaped. Monocytes migrate from blood to tissues and develop into macrophages and dendritic cells. Macrophages are large cells present in nearly all tissues. They actively perform phagocytic functions. Dendritic cells are commonly found in tissue located in areas that come in contact with antigens from the external environment. They are found in the skin, internally in the nose, lungs, and gastrointestinal tract. Dendritic cells function primarily to present antigenic information to lymphocytes in lymph nodes and lymph organs. This aids in the development of antigen immunity. Dendritic cells are so named because they have projections that are similar in appearance to the dendrites of neurons.
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  White Blood Cell Production

  White blood cells are produced by bone marrow within bone. Some white blood cells mature in the lymph nodes, spleen, or thymus gland. The life span of mature leukocytes ranges from about a few hours to several days. Blood cell production is often regulated by body structures such as the lymph nodes, spleen, liver, and kidneys. During times of infection or injury, more white blood cells are produced and are present in the blood. A blood test known as a WBC or white blood cell blood cells in the blood. Normally, there are between 4,300-10,800 white blood cells present per microliter of blood. A low WBC count may be due to disease, radiation exposure, or bone marrow deficiency. A high WBC count may indicate the presence of an infectious or inflammatory disease, anemia, leukemia, stress, or tissue damage.
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  Other Blood Cell Types

• Red Blood Cells - these biconcave-shaped cells transport oxygen to the cells and tissues of the body through blood circulation. They also transport carbon dioxide to the lungs.
• Platelets - these blood cells are vital for the clotting process, which is needed to prevent blood loss.
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