White blood cells / Leukocytes / White cells (WBC)
White cells, or leukocytes, exist in variable numbers and types but make up a very small part of blood's volume--normally only about 1% in healthy people. Leukocytes are not limited to blood. They occur elsewhere in the body as well, most notably in the spleen, liver, and lymph glands. Most are produced in our bone marrow from the same kind of stem cells that produce red blood cells. Others are produced in the thymus gland, which is at the base of the neck. Red cells remain viable for only about 4 months before they are removed from the blood and their components recycled in the spleen. Individual white cells usually only last 18-36 hours before they also are removed, though some types live as much as a year. If a white blood cell count is too high, it indicates that the person has some type of infection. There are actually many specialized sub-types of them that participate in different ways in our immune responses.
They can be divided into granulocytes and agranulocytes. The former have cytoplasm that contains organelles that appear as colored granules through light microscopy, hence their name. Granulocytes consist of Neutrophils, eosinophils and Basophils.
In contrast, agranulocytes do not contain granules and consist of lymphocytes and Monocytes.
Production of WBC’s: -
Leucopoiesis refers to the production of leukocytes (WBCs). It begins when some types of haemocytoblast differentiate into three types of committed cells:
These cells have receptors for colony-stimulating factors (CSFs). Each CSF stimulates a different WBC type to develop in response to specific needs. Mature lymphocytes and macrophages secrete several types of CSFs in response to infections and other immune challenges. The red bone marrow stores granulocytes and Monocytes until they are needed in the bloodstream. However, circulating leukocytes do not stay in the blood for very long. Granulocytes circulate for 4-8 hours and then migrate into the tissues where they live for another 4-5 days. Monocytes travel in the blood for 10-20 hours, then migrate into the tissues and transform into a variety of macrophages which can live as long as a few years. Lymphocytes are responsible for long-term immunity and can survive from a few weeks to decades. They are continually recycled from blood to tissue fluid to lymph and finally back to the blood.
White cells, or leukocytes, exist in variable numbers and types but make up a very small part of blood's volume--normally only about 1% in healthy people. Leukocytes are not limited to blood. They occur elsewhere in the body as well, most notably in the spleen, liver, and lymph glands. Most are produced in our bone marrow from the same kind of stem cells that produce red blood cells. Others are produced in the thymus gland, which is at the base of the neck. Red cells remain viable for only about 4 months before they are removed from the blood and their components recycled in the spleen. Individual white cells usually only last 18-36 hours before they also are removed, though some types live as much as a year. If a white blood cell count is too high, it indicates that the person has some type of infection. There are actually many specialized sub-types of them that participate in different ways in our immune responses.
They can be divided into granulocytes and agranulocytes. The former have cytoplasm that contains organelles that appear as colored granules through light microscopy, hence their name. Granulocytes consist of Neutrophils, eosinophils and Basophils.
- Basophils: - They have a pale nucleus that is usually hidden by granules. They secrete histamine which increases tissue blood flow via dilating the blood vessels, and also secrete heparin which is an anticoagulant that promotes mobility of other WBCs by preventing clotting.
- Eosinophils: - They eat other cells. The technical term for the eating of a cell is phagocytosis, so eosinophils are said to phagocytize complexes formed between antigens and antibodies. These have large granules and a prominent nucleus that is divided into two lobes. They function in the destruction of allergens and inflammatory chemicals, and release enzymes that disable parasites.
- Neutrophils: - They are the most abundant white blood cells in the body. These contain very fine cytoplasmic granules that can be seen under a light microscope. They are also called polymorphonuclear (PMN) because they have a variety of nuclear shapes. These cells phagocytize bacteria, and in doing so keep your system from being overrun by every germ with which it come into contact. They play roles in the destruction of bacteria and the release of chemicals that kill or inhibit the growth of bacteria.
In contrast, agranulocytes do not contain granules and consist of lymphocytes and Monocytes.
- Lymphocytes: - They kill cells that contain viruses. Lymphocytes scan the body looking for viruses. These are usually classified as small, medium or large. Medium and large lymphocytes are generally seen mainly in fibrous connective tissue and only occasionally in the circulation bloodstream. Lymphocytes function in destroying cancer cells, cells infected by viruses, and foreign invading cells. In addition, they present antigens to activate other cells of the immune system. They also coordinate the actions of other immune cells, secrete antibodies and serve in immune memory. There are two types of lymphocytes: B cells and T cells.
- Monocytes: - They are precursors to macrophages, meaning “big eater.” Macrophages digest bacteria and viruses. They are the largest of the formed elements. Their cytoplasm tends to be abundant and relatively clear. They function in differentiating into macrophages, which are large phagocytic cells, and digest pathogens, dead Neutrophils, and the debris of dead cells. Like lymphocytes, they also present antigens to activate other immune cells.
Production of WBC’s: -
Leucopoiesis refers to the production of leukocytes (WBCs). It begins when some types of haemocytoblast differentiate into three types of committed cells:
- B progenitors, which are destined to become B lymphocytes
- T progenitors, which become T lymphocytes
- Granulocyte-macrophage colony-forming units, which become granulocytes and Monocytes
These cells have receptors for colony-stimulating factors (CSFs). Each CSF stimulates a different WBC type to develop in response to specific needs. Mature lymphocytes and macrophages secrete several types of CSFs in response to infections and other immune challenges. The red bone marrow stores granulocytes and Monocytes until they are needed in the bloodstream. However, circulating leukocytes do not stay in the blood for very long. Granulocytes circulate for 4-8 hours and then migrate into the tissues where they live for another 4-5 days. Monocytes travel in the blood for 10-20 hours, then migrate into the tissues and transform into a variety of macrophages which can live as long as a few years. Lymphocytes are responsible for long-term immunity and can survive from a few weeks to decades. They are continually recycled from blood to tissue fluid to lymph and finally back to the blood.