White blood cell

White blood cells (WBCs) are one of the four main components of blood, along with red blood cells (RBCs), platelets, and plasma. WBCs are also called leukocytes. As part of the body's immune system, WBCs are primarily responsible for defending the body from disease and infection. The body has five main types of WBCs, each designed to protect the body from a particular threat. An abnormally high or low number of WBCs in the blood is usually a symptom of another problem.

Background

Blood is the red liquid that transports nutrients and oxygen to cells in the body's tissues and organs. It collects waste products from these cells and carries them away for elimination. Blood circulates throughout the body in a series of blood vessels including arteries, veins, and capillaries. Arteries generally carry blood with a high oxygen content, while veins generally carry blood with a low oxygen content. Capillaries are the smallest of the body's blood vessels. They serve as sites where cells exchange waste products for nutrients and oxygen.

The four main components of blood are plasma, RBCs, WBCs, and platelets. Plasma is the liquid component of blood that contains the RBCs, WBCs, and platelets. About half of a person's total blood volume is plasma. Plasma is a combination of water, sugars, proteins, and other materials. Plasma's main function is to transport blood cells and platelets throughout the body along with nutrients, hormones, and other materials that the body needs to function.

RBCs, or erythrocytes, account for another 40 to 45 percent of blood volume. RBCs contain the protein hemoglobin, which helps carry oxygen to cells and carbon dioxide away from cells. Blood's red color comes from the hemoglobin in RBCs.

WBCs, or leukocytes, are less numerous than RBCs. WBCs make up about 1 percent of total blood volume, although a single drop of blood may contain between seven thousand and twenty-five thousand WBCs. These blood cells are an integral part of the body's immune system, fighting disease and infection. The five types of WBCs are neutrophils, lymphocytes, monocytes, eosinophils, and basophils. Each type performs a specific function.

Platelets, or thrombocytes, are less plentiful than RBCs but more plentiful than WBCs. The blood has about twenty RBCs for every one platelet. Platelets are cell-like structures (but not actually cells) that aid in blood clotting. Platelets gather at a wound site and stick to the lining of a blood vessel to form a clot that stops the vessel from leaking. The clot becomes the foundation for new tissue to form as the wound heals.

Overview

WBCs are produced in the bone marrow, a spongy material inside the bones, from hematopoietic stem cells through a continuous process called hematopoiesis. Unlike RBCs, which have a life span of more than one hundred days, WBCs typically last in the body for just three to four days. Some last for only a few hours. The ratio of WBCs to RBCs in the blood is about 1 to 600 or 700. Nevertheless, the body contains about fifty billion WBCs at any given time.

When viewed under a microscope, some WBCs have visible granules, or grains, in their cytoplasm. These WBCs are called granulocytes. WBCs without these visible granules are called agranulocytes. The granulocytes include basophils, eosinophils, and neutrophils. The agranulocytes include lymphocytes and monocytes. A large majority, as much as 70 percent, of the WBCs in the body are neutrophils. Lymphocytes are the second most common type.

Neutrophils live for about a day. Their primary function is to find and destroy bacteria and fungi. Neutrophils flock to the site of an abrasion or infection to prevent germs from entering the bloodstream. Neutrophils use a process called phagocytosis to kill invading germs. During this process, the neutrophil identifies a bacterial or fungal cell as a foreign invader, engulfs it, and digests it. In infected wounds, a yellowish substance called pus indicates the presence of thousands of neutrophils.

Lymphocytes are further divided into three types: T lymphocytes (T cells), natural killer cells, and B lymphocytes (B cells). Both T cells and natural killer cells are designed to directly attack harmful cells, including some cancer cells. A natural killer cell, for example, attaches itself to a cancer cell and injects it with a poison capable of destroying it from within. B cells make antibodies. Antibodies remember a particular pathogen responsible for an illness or disease. The next time that pathogen enters the body, the antibodies are able to identify it more quickly, and T cells are able to more easily target it for destruction.

In terms of size, monocytes are the biggest WBCs, and they tend to last the longest. Monocytes eventually transform into macrophages, which, like neutrophils, use phagocytosis to digest harmful materials in the body. Macrophages eliminate dead cells, cell waste, and bacteria. Once macrophages engulf their "prey," they release special enzymes to destroy it.

Eosinophils mainly target parasites and worms but also help with allergic reactions. As granulocytes, eosinophils have granules in their cytoplasm. These granules contain toxins that, when released, can help to eliminate parasites and other harmful pathogens.

Basophils are the least numerous WBCs in the blood. These WBCs are associated with allergic reactions and show up almost immediately when harmful pathogens enter the body. One of their main functions is to increase blood flow so other WBCs can get to the site of an infection more quickly.

Although WBCs defend the body from viruses, disease, and infection, an unusually high number of WBCs in the bloodstream, called leukocytosis, is cause for concern. A high WBC count can mean that the body is actively fighting an infection or a disease, or is experiencing an allergic reaction. A high WBC count is also a symptom of a form of blood cancer called leukemia. A low WBC count, or leukopenia, is a sign that the body's immune system has been compromised. Cancer treatments such as chemotherapy and radiation are designed to destroy cancer cells, but they can decimate other fast-growing cells in the body, including WBCs and RBCs, in the process and lead to leukopenia. Certain medications and illnesses such as HIV/AIDS can cause leukopenia, too. As a result, the body becomes vulnerable to infection because it has too few WBCs to defend against infectious agents.

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