Leukopoiesis (LOO-co-poy-EE-sis) is the production of white blood cells (see fig. 18.4). It begins when some hemocytoblasts differentiate into three types of committed cells:
1. B progenitors, destined to become B lymphocytes;
2. Tprogenitors, which become T lymphocytes; and
3. granulocyte-macrophage colony-forming units, which become granulocytes and monocytes.
These committed cells have receptors for colony-stimulating factors (CSFs). Mature lymphocytes and macrophages secrete several types of CSFs in response to infections and other immune challenges. Each CSF stimulates a different WBC type to develop in response to specific needs. Thus, a bacterial infection may trigger the production of neu-trophils whereas an allergy triggers the production of eosinophils, each process working through its own CSF.
The red bone marrow stores granulocytes and mono-cytes until they are needed and contains 10 to 20 times more of these cells than the circulating blood does. Lymphocytes begin developing in the bone marrow but do not stay there. Some types mature there and others migrate to the thymus to complete their development. Mature lymphocytes from both locations then colonize the spleen, lymph nodes, and other lymphoid organs and tissues.
Circulating leukocytes do not stay in the blood for very long. Granulocytes circulate for 4 to 8 hours and then migrate into the tissues, where they live another 4 or 5 days. Monocytes travel in the blood for 10 to 20 hours, then migrate into the tissues and transform into a variety of macrophages (MAC-ro-fay-jes). Macrophages can live as long as a few years.
Lymphocytes, responsible for long-term immunity, survive from a few weeks to decades; they leave the bloodstream for the tissues and eventually enter the lymphatic system, which empties them back into the bloodstream. Thus, they are continually recycled from blood to tissue fluid to lymph and finally back to the blood. The biology of leukocytes and macrophages is discussed more extensively in chapter 21.
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