The intact skin forms an effective mechanical barrier to invasion by microorganisms partly because it is composed of tightly associated epithelial cells covered by a highly cross-linked keratin layer. As noted, the skin possesses an array of antimicrobial properties that form a protective shield consisting of a battery of broad-spectrum defensive chemicals, principally peptides, synthesized as precursors and processed by specific proteases into mature active forms that target microbial membranes in a manner similar to disinfectants. These help keep pathogenic organisms out, and the microbiota are also checked by these functions. The antimicrobial peptides include cathelicidins, β-defensins, and dermicidin.^{22,23,29,31–34} Not only do they function directly to kill microbes, they also act as chemokines to attract migrating phagocytic cells. β-Defensin, LL-37 (an antimicrobial peptide), cathelicidin levels, and hexadecenoic acid are depressed in atopic skin, a condition in which microbial suprainfection is common.^35,36

The relative dryness and mild acidity (the so-called acid mantle, pH 5 to 6) of skin, combined with normal skin microbiota act to form an effective prohibitive environment. Inflamed skin is more permeable to water and is more hospitable to colonization. It has been speculated that oily skin may retard evaporation of water, resulting in increased numbers of colonizing microorganisms. The acidity of the skin results from the breakdown of lipids into fatty acids by skin microbiota. Sebum triglycerides are partially hydrolyzed, liberating fatty acids. The continual desquamation of skin scales also aids in the elimination of microorganisms. Because few organisms have the ability to penetrate the skin, they usually gain access by some physical means, such as an arthropod vector, trauma, surgical incision, or intravenously placed catheter.

Mucous Membranes

Because of the inherent moisture with which they are associated, mucous membranes support a broader spectrum and larger number of microorganisms than does skin. Most epithelial cells possess the same peptide shield as skin (see Table 4-2)^29,37,38; however, body secretions, including saliva, cervical mucus, prostatic fluid, and tears, are endowed with unique antimicrobial properties. Two of the more potent antimicrobial substances are lysozyme and N-acetylmuramyl-l-alanine amidase (NAMLAA). Both substances are particularly effective against gram-positive bacteria because they hydrolyze the amino acid backbone of peptidoglycan.³⁹

Local secretions also contain immunoglobulins, principally IgG and secretory IgA, which act primarily to agglutinate microorganisms, block the attachment of organisms to receptors on host cells com­petitively, or both. The body produces more secretory IgA than total IgG. The IgA immunoglobulin is made relatively resistant to proteolysis by binding with a unique polypeptide known as the secretory component and protein Fv.⁴⁰ There is also evidence that mucosal IgA can bind intracellular pathogens or products as they are transported through the cell.⁴¹

Mucosal secretions also contain significant amounts of iron-binding proteins. Iron is a critical element for most microorganisms, and fluids that potentially are exposed to microbes are enriched with iron-binding proteins that act to keep this important factor from the microorganisms. In contrast, microorganisms that routinely colonize skin and mucosal surfaces have evolved mechanisms to acquire iron despite these proteins.^42,43

Respiratory Tract

The respiratory tract has a formidable array of antimicrobial defense mechanisms. The inhaled particles must survive and penetrate the aerodynamic filtration system of the upper airway and tracheobronchial tree. The airflow in these areas is turbulent, causing large particles to come in contact with the mucosal surfaces and face the full array of those defense mechanisms. Humidification also causes hygroscopic organisms to increase in size, aiding trapping.

When a particle is deposited, the mucociliary blanket transports the invading offender away from the lung. Coughing aids this expulsion. This system is amazingly efficient; 90% of deposited material is cleared in less than 1 hour.⁴⁴ In addition, the bronchial secretions contain various antimicrobial substances, such as lysozyme, NAMLAA, and β-defensins such as hBD-1 and hBD-2,⁴⁵ RNase 7 and pulmonary lectins,⁴⁶ and surfactant collectins.^38,46,47 The airway epithelium also contributes to the resistance to infection. On exposure to pathogens, lung epithelial cells express S100A, antileukoprotease,²⁹ and large amounts of major vault protein (MVP). In cells infected with Pseudomonas aeruginosa, MVP I localizes in lipid raft sections of cell membranes and helps mediate bacterial clearance.⁴⁸

After a particle reaches the alveoli, physical expulsion becomes much less effective and the alveolar macrophages and tissue histiocytes play a more prominent role in protecting the host. These phagocytic cells are assisted in their defense by the collectin surfactants SP-A and SP-D, which bind to and opsonize diverse organisms, including gram-negative bacteria, fungi, and Pneumocystis jirovecii (formerly known as Pneumocystis carinii).⁴⁹

Like all defense mechanisms, these nonspecific mechanisms can be overcome by the introduction of large numbers of invading organisms (e.g., a contaminated respirator), particularly if the host is exposed for an extended period. Their effectiveness is decreased by air pollutants (e.g., cigarette smoke), mechanical respirators, tracheostomy, concomitant infection, allergenic agents, and, in some cases, genetic defects (e.g., cystic fibrosis) and inhibitory factors of some pathogens.^47,50

Intestinal Tract

The acid pH of the stomach and the antibacterial effect of the various pancreatic enzymes, bile, and intestinal secretions are effective, nonspecific, antimicrobial defense factors. Paneth cells of the small intestine, located in the crypts of Lieberkühn, secrete antimicrobial substances, such as β-defensins, lysozyme, and type II phospholipase A.^2,51 As with other mucosal surfaces, the intestinal tract expresses microbial pattern recognition receptors on a number of cell types.⁵² Understanding how these components work to exclude pathogens while maintaining commensal species is important for maintaining the health of the gut. For example, the development of natural immunity, the cross-reacting antibodies against blood group antigens, and potential pathogens such as Neisseria meningitidis is dependent on this process. The microbiome has been shown to keep the intestinal environment in a constant state of “appropriate” immune activation, thus triggering antimicrobial proteins, tissue repair factors, and immunoglobulin A, resulting in the maintenance of the intestinal barrier and gut immunity.^2,51,53 Microbial colonization in germ-free mice triggers the expression of a secreted C-type lectin with known antibacterial properties. The protein in humans, HIP/PAP (hepatocarcinoma-intestine-pancreas/pancreatic-associated protein),⁵⁴ represents a form of innate response to help maintain a symbiotic relationship.⁵⁵ Peristalsis and the normal loss of epithelial cells also act to purge the intestinal tract of harmful microorganisms. Alteration of these parameters can lead to increased susceptibility of the host to infection. For example, Salmonella and Mycobacterium tuberculosis infections are more common in achlorhydric patients and slowing peristalsis with belladonna or opium alkaloids prolongs symptomatic disease caused by gastrointestinal pathogens such as Shigella species. A number of disease states have been shown to be accompanied by the movement, or translocation, of gram-negative bacterial products across the epithelial barrier and into the circulation.⁵⁶ This is often accompanied by a loss of tight junctions between enterocytes or the loss of enterocytes altogether. This may be tied to loss or impairment of a subset of T cells, Th17 cells, which secrete the enterocyte growth factor cytokine Th17.⁵⁷

The importance of a symbiotic intestinal microbiome has long been appreciated and has been highlighted by the success of fecal transplants in patients with intractable Clostridium difficile infections.⁵⁸ The normal bowel microbiota compete for nutrients (10¹² organisms/g of feces), and this plays a crucial protective role. Altering this symbiotic normal intestinal microbiota, such as with broad-spectrum antibiotics, can lead to overgrowth of inherently pathogenic organisms (e.g., Salmonella typhimurium) or suprainfection with ordinarily commensal organisms (e.g., Candida albicans). As always, the interfering competitive capacity of the normal microbiota can be overcome by the introduction of large numbers of pathogenic organisms. The probability of development of salmonellosis is related directly to the number of Salmonella organisms ingested. One exception to this is Shigella, which requires very few organisms to establish infection.

Genitourinary Tract

Urine has been considered sterile. Evidence is emerging that there is a spectrum of uncultivated bacteria present in the bladders of adult females.⁵⁹ The contribution of these organisms to keeping the bladder pathogen free remains to be determined. In this regard, urine is bactericidal for some strains of bacteria, mostly because of its pH, although factors such as hypertonicity, urea, and other solutes play a role. Tamm-Horsfall protein is a glycoprotein produced by the kidneys and excreted in large amounts in urine (approximately 50 mg/L). Because many bacteria are avidly bound to it, the protein acts as a sponge and is a natural host defense mechanism against tissue colonization and subsequent infection by preventing these bacteria from gaining a foothold on the cellular lining of the urinary tract.⁶⁰

The lower urinary tract is rinsed with urine four to eight times each day, eliminating potential pathogenic organisms, unless they are capable of firmly attaching to epithelial cells of the urinary tract (e.g., Neisseria gonorrhoeae, certain strains of E. coli). Urinary retention or lack of complete bladder emptying impedes this flushing process.

The length of the male urethra (20 cm in an adult) also provides passive protection, and bacteria seldom gain access to the bladder in men unless introduced by instrumentation. The female urethra is much shorter (5 cm in an adult) and is traversed more readily by microorganisms, which is one reason why urinary tract infections are 14 times more common in women than in men.

The hypotonic state of the kidney medulla presents an unfavorable milieu for most microorganisms. This can be decreased by increased glucose urine levels secondary to hyperglycemia. This is a factor that accounts for the increased incidence of pyelonephritis in diabetic patients.

The vagina has an additional unique mechanism of protection. Under hormonal influence, especially estrogen, the vaginal epithelium contains increased amounts of glycogen that Döderlein’s bacilli and other commensals metabolize into lactic acid. Döderlein’s bacilli is an all-encompassing term used to describe acidogenic gram-positive rods, especially lactobacilli, residing in the vagina. Normal vaginal secretions contain 10⁸/mL of these bacteria. They establish an acid environment that is unfavorable to most pathogenic microorganisms. The vaginal secretions of women with nonspecific vaginitis or vaginosis, which is a perturbation of the normally protective vaginal ecosystem, and of patients with other sexually transmitted diseases are characterized by an elevated pH.⁶¹

Eye

Constant bathing of the eye by tears is an effective means of protection. Foreign substances continually are diluted and washed away via the tear ducts into the nasal cavity. Tears also contain large amounts of lysozyme, lactoferrin, and lipocalin.⁶²

Chemokines and Chemotaxis

Critical to the innate response is the ability of leukocytes to move to the site of pathogen invasion. Two main families of chemokines, part of the first set of response factors induced by the innate immune system, direct such leukocyte migration.^71,72 Members of the CXC family, whose first two cysteines are separated by a single amino acid, stimulate chemotaxis of polymorphonuclear neutrophils (PMNs), eosinophils, monocytes, dendritic cells, natural killer (NK) cells, and T lymphocytes. Members of the CC family, whose first two cysteines adjoin each other, stimulate all these leukocytes except PMNs, which do not express receptors for CC chemokines. More than 45 chemokines, 11 receptors for CC chemokines, and 6 receptors for CXC chemokines have been described. The defect in the chemokine receptor CCR5 is best known for providing resistance to HIV infection. However, loss of this chemokine receptor has also been shown to be associated with increased susceptibility to symptomatic West Nile virus infection.⁷³ This effect is related in some way to the loss of known chemokine effects on upregulation of adhesion molecules, stimulation of leukocyte migration, and lowering the threshold for T-cell activation. Cytokines, especially tumor necrosis factor-α and interleukin (IL)-1, also increase expression of adhesion molecules on endothelial cells, PMNs, NK cells, and monocytes, which aid in the binding and transmigration of these cells into sites of inflammation.

The interactions of chemokines on target cell populations are extraordinarily complex. Resting T cells express few chemokine receptors, but these are strongly upregulated by IL-2 in conjunction with stimulation via the antigen receptor. The most vigorous immigration of T cells to inflammatory sites occurs in conjunction with acquired immunity rather than during the innate response.