bacterial cell is enclosed. The antimicrobial factors kill the immobilized microbes.
There are two types of antimicrobial factors, those that are preformed inside the phagocyte and one that is induced in response to the phagocytic process. The most important of the antimicrobial mechanisms is the respiratory burst, which generates oxygen radicals—superoxide, hydroxy l radicals, and hydrogen peroxide. The respiratory burst is the only induced mechanism. All of the active oxygen species are highly destructive to bacterial as well as host cells, so they are not produced until they are needed. The defensins are arginine- or cysteine-rich bactericidal peptides that exhibit an extremely broad spectrum of antimicrobial activity. The defensins will kill bacteria (Gram-positive and Gram-negative), fungi, and even some viruses. The mechanism of action of the defensins is unknown, but because the peptides are highly charged in an opposite sense to bacterial cell membranes, an electrostatic, membrane-disruptive interaction might be involved. Bacteria have an absolute requirement for iron, and to compete with the host for this element, they secrete high-affinity siderophore factors that scavenge iron from the host's stores. Lactoferrin is a substance produced by the host that binds iron more tightly than the bacterial chelator, preventing the invading organism's access to a critical nutrient. Lysozyme is an important component of the antimicrobial system. This enzyme hydrolyzes [1-4]-glycosidic bonds, as in the peptidoglycan of bacterial cell walls. Lysozyme is present in almost all body fluids, including tears and saliva.
Hepatocytes produce an array of acute phase proteins (Table 5.4) that are released into the serum during inflammation or infection. These proteins do not act directly on bacteria, but they augment the bactericidal activity of other antimicrobial factors.
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