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The immune system protects the body from invading pathogens and has an amazing ability to evolve and adapt based upon environmental exposure. The immune system is designed to attack and destroy foreign antigens/pathogens; however, the immune system must be able to distinguish self from nonself. Failure to differentiate self from nonself may lead to autoimmune diseases1 (see Table 11-1 for a list of autoimmune diseases). The immune system includes two functional divisions: (1) innate or nonspecific and (2) adaptive or specific.2, 3 The body uses the innate and adaptive immune responses to kill foreign pathogens. The greatest differences between the responses are in specificity and memory. The adaptive immune response can evolve with each subsequent infection, whereas the innate immune response stays the same with each infection.2, 3 Awareness of immune systems components and consequences of disrupting homeostasis must be understood in order to appropriately dose, administer, and monitor effects of medications given to manipulate immune responses.

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TABLE 11-1 Autoimmune Diseases
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Physical and chemical defenses compose the innate immune system and are the first line of defense against pathogens.

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Physical Defense

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The skin is the primary method of physical defense. Alterations in the skin allow for an easy portal of entry for pathogens.4 Burns and abrasions are common examples that alter the physical defense of the skin; however medications can also alter this nonspecific system (drug-associated Stevens Johnson syndrome—see Table 11-2). The low pH of the stomach serves as a major defense to pathogen entry through the gastrointestinal system. Medications that alter the pH of the stomach may change the gastrointestinal bacterial flora and increase risk of infections. Antisecretory agents such as proton pump inhibitors have been associated with pH changes in the stomach and subsequent development of bacterial infections.5 The rapid turnover of gastrointestinal cells also limits systemic infection, as cells are frequently sloughed. Cell-cycle antineoplastics that disrupt the sloughing process may leave the patient at increased risk for infections. The respiratory tract has forms of physical defense such as coughing, mucous coating the epithelial cells, and the cilia lining the epithelium of the lungs. The combination of coughing, cilia, and mucus provide a barrier to invasion of the respiratory tract. Disruption of the respiratory physical defense through mechanical ventilation can increase the risk for penetration by a pathogenic organism (pneumonia) or anti-infectives that alter gastrointestinal flora leaving the patient at an increased risk of infection (eg, Clostridium difficile infection)....

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