white blood cell or leukocytes

WBC / Leukocytes blood cells 

white blood cell

             



 Definition : any of the blood cells that are colorless, lack hemoglobin, contain a nucleus, and include the lymphocytes, monocytes, neutrophils, eosinophils, and basophils —called also leukocyte, white blood corpuscle, white cell. all leukocytes are product and derivcd from a multipotent cell in the bone marrow as a hematopoietic stem cell . leukocyted are found through the body including the blood and lymphatic cell .  

Five Types of White Blood Cells and Their Functions

There are two different types of white blood cells and each looks different from one another under the microscope. These include granulocytes and agranulocytes.

• ŸGranulocytes have visible granules or grains inside the cells that have different cell functions. Types of granulocytes include basophils, neutrophils, and eosinophils.
• ŸAgranulocytes are free of visible grains under the microscope and include lymphocytes and monocytes.

Together, they coordinate with one another to fight off things like cancer, cellular damage, and infectious diseases. Below, detailed information about each type will be discussed.

1.  Neutrophils

Neutrophils are the most common type of white blood cell in the body with levels of between 2000 to 7500 cells per mm³ in the bloodstream. Neutrophils are medium-sized white blood cells with irregular nuclei and many granules that perform various functions within the cell.
Function: Neutrophils function by attaching to the walls of the blood vessels, blocking the passageway of germs that try to gain access to the blood through a cut or infectious area. Neutrophils are the first cells to reach an area where a breach in the body has been made. They kill germs by means of a process known as phagocytosis or “cell-eating”. Besides eating bacteria one-by-one, they also release a burst of super oxides that have the ability to kill many bacteria at the same time.

2.  Lymphocytes

Lymphocytes are small, round cells that have a large nucleus within a small amount of cytoplasm. They have an important function in the immune system, being major players in the humoral immune system, which is the part of the immune system that relates to antibody production. Lymphocytes tend to take up residence in lymphatic tissues, including the spleen, tonsils, and lymph nodes. There are about 1300 to 4000 lymphocytes per mm³ of blood.
Function: B lymphocytes make antibodies, which is one of the final steps in disease resistance. When B lymphocytes make antibodies, they prime pathogens for destruction and then make memory cells ready that can go into action at any time, remembering a previous infection with a specific pathogen. T lymphocytes are another type of lymphocyte, differentiated in the thymus and important in cell-mediated immunity.

3.  Monocytes

Monocytes are the largest of the types of white blood cells. There are only about 200-800 monocytes per mm³ of blood. Monocytes are agranulocytes, meaning they have few granules in the cytoplasm when seen under the microscope. Monocytes turn into macrophages when they exit the bloodstream.
Function: As macrophages, monocytes do the job of phagocytosis (cell-eating) of any type of dead cell in the body, whether it is a somatic cell or a dead neutrophil. Because of their large size, they have the ability to digest large foreign particles in a wound unlike other types of white blood cells.

4.  Eosinophils

There aren’t that many eosinophils in the bloodstream—only about 40-400 cells per mm³ of blood. They have large granules that help in cellular functions. Eosinophils are especially important when it comes to allergies and worm infestations.
Function: Eosinophils work by releasing toxins from their granules to kill pathogens. The main pathogens eosinophils act against are parasites and worms. High eosinophil counts are associated with allergic reactions.

5.  Basophils

Basophils are the least frequent type of white blood cell, with only 0-100 cells per mm³ of blood. Basophils have large granules that perform functions that are not well known. They are very colorful when stained and looked at under the microscope, making them easy to identify.
Function: Basophils have the ability to secrete anticoagulants and antibodies that have function against hypersensitivity reactions in the bloodstream. They act immediately as part of the immune system’s action against foreign invaders. Basophils contain histamine, which dilates the vessels to bring more immune cells to the area of injury.

                                         

Phagocytosis

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Several types of cells in the immune system engulf microorganisms via phagocytosis.

• Neutrophils. Neutrophils are abundant in the blood, quickly enter tissues, and phagocytize pathogens in acute inflammation.
• Macrophages. Macrophages are closely related to monocytes in the blood. These longer-lived cells predominate in chronic inflammation. They also release some important inflammatory paracrines. (See below.)
• Dendritic Cells. Phagocytosis in these cells is important for the elaboration of a specific immune response rather than for directly destroying the pathogens.
• B Lymphocytes. A small amount of phagocytosis in these cells is often necessary in order for them to develop into cells that release antibodies.

Sequence of Events

Phagocytosis begins with the neutrophil or macrophage flowing around the pathogen and engulfing it so that it winds up enclosed in a phagosome (phagocytic vesicle). But this is only the first step, because the more challenging task of destroying the microorganisms remains. Indeed, some pathogens have special, effective mechanisms for frustrating this destruction step.
The next step is the fusion of lysosomes with the phagosome. The result is called a phagolysosome. Lysosome are derived from the Golgi apparatus, much like secretion vesicles, but their contents are focused on destroying microorganisms.

Destruction of the Microbes

The following are important factors that help destroy microorganisms within a phagolysosome:

• Oxygen Radicals. A complex of proteins called phagocyte oxidase in the membrane of a phagolysosome generates oxygen radicals in the phagosome. A single electron is taken from NADPH and added to oxygen, partially reducing it. The resulting highly reactive molecules react with proteins, lipids and other biological molecules. See the next webpage for details.
• Nitric Oxide. Nitric oxide synthase synthesizes nitric oxide, a reactive substance that reacts with superoxide to create further molecules that damage various biological molecules. (But nitric oxide is also, remarkably enough, an important regulatory molecule elsewhere. More on this later this quarter.)
• Anti-Microbial Proteins. Lysosomes contain several proteases, including a broad spectrum enzyme,elastase, which is important or even essential for killing various bacteria. Another anti-microbial protein is lysozyme, which attacks the cell walls of certain (gram positive) bacteria.
• Anti-Microbial Peptides. Defensins and certain other peptides attack bacterial cell membranes. Similar molecules are found throughout much of the animal kingdom.
• Binding Proteins. Lactoferrin binds iron ions, which are necessary for growth of bacteria. Another protein binds vitamin B12.
• Hydrogen Ion Transport. Transporters for hydrogen ions (a second role of the oxidase) acidify the phagolysosome, which kills various microorganisms and is important for the action of the proteases described above.

Release of Regulatory Molecules

In addition to destroying the microorganism, phagocytes also release molecules that diffuse to other cells and help coordinate the overall response to an infection.
Regulatory molecules that regulate an immune response are called cytokines. Most are small proteins and are mainly released by white blood cells and their relatives, such as macrophages.
Cytokines for the most part act as paracrines, which are regulatory molecules that are released by one cell and diffuse locally to neighboring cells. (Locally in this context means over millimeters or perhaps a few centimeters). But occasionally cytokines act more widely. For example, certain cytokines diffuse from a site of infection and cause fever.
The release of cytokines by macrophages is especially important. Two important examples are TNF-alpha and IL-1. These help coordinate an immune response. But they are especially important as potent imflammatory paracrines. Also, you are going to find that TNF-alpha can be released in serious infections in such quantity that it is life-threatening or even fatal.

Identification of Pathogen

We will go into this topic in more detail later. But here are a few points for now. Neutrophils and macrophages have some ability on their own to recognize microorganisms and begin phagocytosis. We will use the term innate receptors for the molecules on such cells that available immediately to bind foreign molecules. These can act as soon as a microbe enters the body. They are naturally found on the surface of phagocytes and do not require a specific immune response to be made. Innate receptors are possible because microorganisms have various molecules on their surfaces that much different than those found in a human.
But phagocytosis is far more effective if microorganisms are labelled by special molecules that bind to their surface. Any molecule that binds to a microorganism and thereby speeds phagocytosis is called an opsonin. Most important here are antibodies (such as IgG), which specifically identify molecules at the surface of specific microorganisms. With this attached to the surface of the microorganisms, phagocytosis is much more effective and rapid.