Some Fundamentals about Blood Cells

Blood is composed of many different kinds of cells, each with a specific function. Most blood cells are formed in the bone marrow and released into the bloodstream at various stages of maturity.

Red blood cells (erythrocytes) make up 45 percent of blood volume. Their primary function is to pick up oxygen in the lungs and transport it to tissues throughout the body. At the tissue site, red blood cells exchange oxygen for carbon dioxide and carry it back to the lungs to be exhaled.

White blood cells leukocytes) are only 1/1,000 as numerous as red blood cells in the bloodstream. There are five main types: neutrophils (also called granulocytes), eosinophils, basophils, monocytes, and Iymphocytes. Each plays a distinct and important role in helping the immune system fight infection.

Neutrophils contain granules of bacteria-killing enzymes in the cytoplasm - the substance surrounding the cell. Eosinophils attack protozoa that cause infection. Basophils are the least common type of white blood cell and their function is not completely understood. They play an important role in regulating allergic reactions such as asthma, hives, hay fever and reactions to drugs.

Monocytes are the largest white blood cells. They engulf and destroy invading bacteria and fungi and clean up debris once foreign organisms have been destroyed by other white blood cells. When monocytes leave the bloodstream and enter tissues or organs, they can evolve into larger cells called macrophages that have an increased capacity to destroy foreign organisms invading the body.

Lymphocytes are the smallest white blood cells and are the backbone of the immune system. Lymphocytes fight viral infections and assist in the destruction of other parasites, bacteria and fungi. One group of lymphocytes called T-cells regulates the immune system's response to invading organisms and is the body's main defense against viruses and protozoa. A second group called B-cells manufactures a kind of protein called an antibody or immunoglobulin. Antibodies attach to the surface of foreign organisms or the cells they have invaded and summon a group of proteins in the Woodstream called the complement system to surround the infected organism or cell and dissolve a hole in it.

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Thrombocytes (platelets) are the smallest cell elements in the bloodstream. Platelets are needed to control bleeding.

BLOOD CELL PRODUCTION

New blood cells are constantly produced by the body. In healthy adults, an estimated 100 billion red cells and 400 million white cells are produced each hour. The life span of mature blood cells is short - only a few days or months.

Ninety-five percent of the body's blood cell production is believed to take place in the bone marrow. The remainder occurs in the spleen. While most blood cells produced in the bone marrow are discharged directly into the bloodstream, T-cells first travel to the thymus gland (thus, the name T cells) where they receive further education or programming before being released into the bloodstream.

All mature blood cells are believed to originate from very primitive cells in the bone marrow called "pluripotent stem cells." This cell is capable of producing other cells identical to itself. Pluripotent stem cells also produce other stem cells the lymphoid stem cell and the myeloid stem cell - from which the various types of mature blood cells evolve.

Like pluripotent stem cells, the myeloid and lymphoid stem cells can self-renew as well as produce colonies of offspring that eventually evolve into mature cells. However, their ability to self-renew is believed to be more limited than that of pluripotent stem cells and they are capable of producing fewer different types of offspring. Lymphoid stem cells only produce cells that evolve into lymphocytes (T-cells or B-cells). The offspring of myeloid stem cells can only evolve into either red blood cells, platelets, or white blood cells other than lymphocytes.

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The myeloid and Iymphoid stem cells produce colonies of "committed progenitor" cells. Unlike stem cells, committed progenitors are only capable of developing into one specific type of mature cell. Cells passing through the final stages of maturation are called precursor cells.

In healthy human beings, the number of each type of stem cell and their offspring is contained within very narrow limits. Certain proteins, such as interleukins and colony-stimulating factors, play a key role in determining whether a stem cell will replicate itself, produce offspring that evolve into mature Wood cells, do both or do neither at any given time. These proteins also regulate the maturation of precursor cells. If this regulatory mechanism breaks down, too many or too few stem cells will be present in the bone marrow and/or certain progenitor or precursor cells will proliferate and fail to properly mature.

In patients with leukemia, for example, one or more types of blood cells (usually white blood cells) fail to properly mature. They stall at one stage of development and self-replicate uncontrollably.

Bone marrow transplants enable physicians to destroy diseased bone marrow with high-dose chemotherapy and/or radiation, and replace it with healthy marrow that will produce normal blood cells. It also enables patients with other malignancies such as breast and ovarian cancer to receive higher than normal doses of chemotherapy to treat their disease. Although the higher doses of chemotherapy destroy bone marrow as well as the tumor, healthy bone marrow can be reinfused after treatment, enabling normal production of blood cells to resume.

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This document was created by NYSERNet, Inc. through a grant funded by the New York State Science and Technology Foundation as part of the Breast Cancer Infomation Clearinghouse.