As described in Chapter 2, neutrophils are circulating granu-locytes with phagocytic function. Quantitative deficiencies in neutrophils can range from an almost complete absence of cells, called agranulocytosis, to a reduction in the concentration of peripheral blood neutrophils below 1500/mm3, called granulocytopenia or neutropenia. These quantitative deficiencies may result from congenital defects or may be acquired through extrinsic factors. Acquired neutropenias are much more common than congenital ones.
Congenital neutropenia is often due to a genetic defect that affects the myeloid progenitor stem cell; it results in reduced production of neutrophils during hematopoiesis. In congenital agranulocytosis, myeloid stem cells are present in the bone marrow but rarely differentiate beyond the promyelocyte stage. As a result, children born with this condition show severe neutropenia, with counts of less than 200 neutrophils/mm3. These children suffer from frequent bacterial infections beginning as early as the first month of life; normal infants are protected at this age by maternal antibody as well as by innate immune mechanisms, including neu-trophils. Experimental evidence suggests that this genetic defect results in decreased production of granulocyte colony-stimulating factor (G-CSF) and thus in a failure of the myeloid stem cell to differentiate along the granulocytic lineage (see Figure 2-1).
Neutrophils have a short life span, and their precursors must divide rapidly in the bone marrow to maintain levels of these cells in the circulation. For this reason, agents such as radiation and certain drugs (e.g., chemotherapeutic drugs) that specifically damage rapidly dividing cells are likely to cause neutropenia. Occasionally, neutropenia develops in such autoimmune diseases as Sjogren's syndrome or systemic lupus erythematosus; in these conditions, autoantibodies destroy the neutrophils. Transient neutropenia often develops after certain bacterial or viral infections, but neutrophil counts return to normal as the infection is cleared.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.