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138 9 Our bodies are continuously bombarded by a variety of infectious pathogens including but not limited to bacteria, fungi, molds, parasites, spores, and viruses. Many of these circulate in the atmosphere as airborne matter. Their concentrations and varieties can vary regionally in any country or clime. But they are also normal inhabitants of the skin, the mouth, the respiratory passages , the GI tract, the urinary tract, and the lining membranes of the eyes. They regularly compromise physiological functions of the cells, tissues, and organs, but when invading deeper body tissues en masse they can and do cause serious pathological conditions including morbidity and mortality. Our body’s main defenses against pathogens are circulating white blood cells (leukocytes, also called WBCs) and residential tissue cells that are derived from WBCs such as tissue macrophages. These systems work in coordination by ingesting and destroying foreign materials through phagocytosis and by creating antibodies and sensitized lymphocytes that can either inactivate or destroy the invaders. Cellular Defenses Immune implies to defend against infection either by specific or nonspecific mechanisms. The word also has general reference to the immune system or to an immune response. One can refer to any good medical dictionary or textbook of immunology and find tables of immunodeficient disorders, diseases, and syndromes . Additionally, the molecular names and structures of the various immunoglubulins in our blood can be found in such books. Leukocytes are the body’s mobile units of defense. They are formed in the bone marrow but also in the lymphatic system. This is one of the needs for healthy bone marrow and healthy lymphatics. The two leukocyte-forming tissues The Immune System are referred to as myeloid (originating in the marrow) and lymphoid (originating in the lymphatics). There are five or six classes of WBCs. Some are characterized by the number of nuclei in the cell, the structure of the nuclei (single-lobed nuclei versus polylobulated nuclei), and their staining qualities (basic, acidic, or neutral). Three classes of polymorphonuclear leukocytes have been described: basophils, eosinophils, and neutrophils. Polymorphonuclear means there are many nuclei of differing shapes. The prefixes baso-, eosino-, and neutro- refer to the colors of the granular, cytosolic elements when they are exposed to different stains. The fourth and fifth classes of WBCs are monocytes and lymphocytes. A sixth class, the plasma cells, is less common. Quantitatively, neutrophils are the most abundant. They constitute about  percent of all WBCs and are followed, quantitatively, by lymphocytes (about  percent), monocytes (about  percent), and the others. Because of their prevalence and from this point forward when referring to granulocytes, I will be considering only neutrophils. Granulocytes are so named because their cytosol contains secretory granules. Once released from the bone marrow into the systemic circulation, granulocytes remain in the vascular compartment for only a few hours. Thereafter and by the process of diapedesis, granulocytes leave the circulation and take up residency in the tissue spaces where they reside for a few days. In times of serious infection, this life cycle is reduced considerably because the granulocytes move rapidly to the site of infection, perform their defensive functions, and are themselves destroyed in the process. Monocytes spend about twice as much time as granulocytes in the circulatory system before they escape into the interstitial spaces. Once outside the vascular compartment, monocytes enlarge considerably to become tissue macrophages. Macrophages can reside in the tissues for weeks or even months unless they are consumed in phagocytic activity shortly after leaving the circulation. The system of monocytes and macrophages has been referred to as the monocyte-macrophage system, the tissue -macrophage system, and the reticuloendothelial system (RES). The last phrase is more entrenched in the medical literature and therefore is more commonly used in general. Lymphocytes originate in the lymph nodes and other tissues of the lymphatic system. They are released along with other lymphatic products into the systemic circulatory system at selected sites such as the subclavian veins of the neck. After a few hours and when they have performed their phagocytic functions in the blood stream, they leave by diapedesis and enter the tissue spaces. Subsequently, they reenter the lymphatic system and are again discharged into the systemic venous system. Thus, lymphocytes get recycled and are therefore able to perform their defensive actions several times during their life cycles of a few weeks to several months. Neutrophils and macrophages are among the main combatants of our immune systems. Neutrophils are mature cells that are able to attack and THE IMMUNE SYSTEM...

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