Abnormally low lymphocyte counts are characteristic of prolonged chronic illness or immunosuppression, including that caused by HIV infection and drug therapies that often involve steroids. Monocytes originate from myeloid stem cells.
They normally represent 2—8 percent of the total leukocyte count. Macrophages are monocytes that have left the circulation and phagocytize debris, foreign pathogens, worn-out erythrocytes, and many other dead, worn out, or damaged cells.
Macrophages also release antimicrobial defensins and chemotactic chemicals that attract other leukocytes to the site of an infection. Some macrophages occupy fixed locations, whereas others wander through the tissue fluid. Abnormally high counts of monocytes are associated with viral or fungal infections, tuberculosis, and some forms of leukemia and other chronic diseases. Abnormally low counts are typically caused by suppression of the bone marrow. Most leukocytes have a relatively short lifespan, typically measured in hours or days.
Production of all leukocytes begins in the bone marrow under the influence of CSFs and interleukins. Secondary production and maturation of lymphocytes occurs in specific regions of lymphatic tissue known as germinal centers. Lymphocytes are fully capable of mitosis and may produce clones of cells with identical properties.
This capacity enables an individual to maintain immunity throughout life to many threats that have been encountered in the past. Leukopenia is a condition in which too few leukocytes are produced.
If this condition is pronounced, the individual may be unable to ward off disease. Excessive leukocyte proliferation is known as leukocytosis.
Although leukocyte counts are high, the cells themselves are often nonfunctional, leaving the individual at increased risk for disease. Leukemia is a cancer involving an abundance of leukocytes. It may involve only one specific type of leukocyte from either the myeloid line myelocytic leukemia or the lymphoid line lymphocytic leukemia. In chronic leukemia, mature leukocytes accumulate and fail to die.
In acute leukemia, there is an overproduction of young, immature leukocytes. In both conditions the cells do not function properly. As in leukemia, the malignant leukocytes do not function properly, and the patient is vulnerable to infection. Some forms of lymphoma tend to progress slowly and respond well to treatment.
Others tend to progress quickly and require aggressive treatment, without which they are rapidly fatal. You may occasionally see platelets referred to as thrombocytes , but because this name suggests they are a type of cell, it is not accurate. A platelet is not a cell but rather a fragment of the cytoplasm of a cell called a megakaryocyte that is surrounded by a plasma membrane.
As noted earlier, thrombopoietin, a glycoprotein secreted by the kidneys and liver, stimulates the proliferation of megakaryoblasts, which mature into megakaryocytes. These remain within bone marrow tissue Figure 3 and ultimately form platelet-precursor extensions that extend through the walls of bone marrow capillaries to release into the circulation thousands of cytoplasmic fragments, each enclosed by a bit of plasma membrane.
These enclosed fragments are platelets. Each megakarocyte releases — platelets during its lifespan. Following platelet release, megakaryocyte remnants, which are little more than a cell nucleus, are consumed by macrophages. After entering the circulation, approximately one-third migrate to the spleen for storage for later release in response to any rupture in a blood vessel. They then become activated to perform their primary function, which is to limit blood loss.
Platelets remain only about 10 days, then are phagocytized by macrophages. Platelets are critical to hemostasis, the stoppage of blood flow following damage to a vessel. They also secrete a variety of growth factors essential for growth and repair of tissue, particularly connective tissue. Infusions of concentrated platelets are now being used in some therapies to stimulate healing.
Thrombocytosis is a condition in which there are too many platelets. He's worked in multiple academic research labs, at a pharmaceutical company, as a TA for chemistry, and as a tutor in STEM subjects. He's currently working full-time as a content writer and editor. Neutrophils Eosinophils Basophils. Which Part of the Body Makes Blood?
Six Types of Neuroglia. Facts About the Spleen. What is Peripheral Blood? What Is Serum? What Is the Primary Function of the Gallbladder? Information on Blood Vessels. Role of the Lungs. Functions of the Human Circulatory System.
Use the links at the bottom of any email to manage the type of emails you receive or to unsubscribe. See our privacy policy for additional details. Learn Site. Monocytes agranular myeloid white blood cells have kidney-shaped nuclei, and they do not have visible granules. Monocytes have a different shape based on whether they are activated or inactivated. Monocytes develop from monoblasts and differentiate into macrophages when they leave the bloodstream and enter the tissues.
Visible Body Biology Learn more. They are colored reddish with general staining techniques. In fresh blood, the red color is provided by erythrocytes because of their high content of hemoglobin, a protein which contains iron in its structure. Erythrocyte, in mammals, can be regarded as a modified cell because it has no nucleus and lacks mitochondria and other cellular organelles.
P latelets are small portions of cytoplasm without nucleus. In blood smears, they appear as small bluish or pale purple aggregates. They are present in mammals, but not in lower vertebrates. Platelets are generated by fragmentation of the cytoplasm of megakaryocytes, a cell type found in the bone marrow.
L eukocytes white blood cells contain a nucleus and are colorless in fresh blood.
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