infectious diseases
Host Defense Mechanisms Against Infection
Natural Barriers Against Infection
The skin usually bars invading microorganisms unless it is physically disrupted (eg, by arthropod vectors, injury, IV catheters, surgical incision). Exceptions include the following: Human Papillomavirus (HPV) Infection , which can invade normal skin, causing warts Some parasites (eg, Schistosomiasis , Many mucous membranes are bathed in secretions that have antimicrobial properties. For example, cervical mucus, prostatic fluid, and tears contain lysozyme, which splits the muramic acid linkage in bacterial cell walls, especially in gram-positive organisms; gram-negative bacteria are protected by lipopolysaccharides in their outer membrane. Local secretions also contain immunoglobulins, principally IgG and secretory IgA, which prevent microorganisms from attaching to host cells, and proteins that bind iron, which is essential for many microorganisms. The respiratory tract has upper airway filters. If invading organisms reach the tracheobronchial tree, the mucociliary epithelium transports them away from the lung. Coughing also helps remove organisms. If the organisms reach the alveoli, alveolar macrophages and tissue histiocytes engulf them. However, these defenses can be overcome by large numbers of organisms, by compromised effectiveness resulting from air pollutants (eg, cigarette smoke), interference with protective mechanisms (eg, endotracheal intubation, tracheostomy), or by inborn defects (eg, Cystic Fibrosis ). Gastrointestinal tract barriers include the acid pH of the stomach and the antibacterial activity of pancreatic enzymes, bile, and intestinal secretions. Peristalsis and the normal loss of intestinal epithelial cells remove microorganisms. If peristalsis is slowed (eg, because of drugs such as belladonna or opium alkaloids), this removal is delayed and prolongs some infections, such as symptomatic Shigellosis and Compromised gastrointestinal defense mechanisms may predispose patients to particular infections (eg, achlorhydria predisposes to Overview of Salmonella Infections , Normal bowel flora can inhibit pathogens; alteration of this flora with antibiotics can allow overgrowth of inherently pathogenic microorganisms (eg,SalmonellaTyphimurium), overgrowth and toxin formation ofC. difficile, or superinfection with ordinarily commensal organisms (eg,Candida albicans). Genitourinary tract barriers include the length of the urethra (20 cm) in men, the acid pH of the vagina in women, the hypertonic state of the kidney medulla, and the urineureaconcentration. The kidneys also produce and excrete large amounts of Tamm-Horsfall mucoprotein, which binds certain bacteria, facilitating their harmless removal. 