326Minnimm - Host Barriers General barriers...

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Host Barriers General barriers Nutrition-malnourished more susceptible Acute phase reactants-iron sequestration and C reactive protein (complement activation, phagocytosis) Fever- “exogenous pyrogen”- microbial component stimulates “endogenous pyrogen (IL-1) -> fever Age- not too young or old Genetic (and species specific) factors Physical barriers Skin- most microbes cannot penetrate unbroken keratinized skin; dry, mildly acidic (pH 4-6), sebum- oily film; washing removes microbes Mucosa- lines the respiratory, digestive and genitourinary systems; traps some microbes; antibacterial substances (e.g. lysozyme); iron binding proteins (e.g. lactoferrin) Respiratory tract- mucociliary “elevator”(coughing and sneezing) brings microbes away from the lungs Gastrointestinal tract- pancreatic enzymes, bile salts; peristalsis Genitourinary tract- flushing action of urine (but short female urethra predisposes to more “UTI’s” Eye- conjunctiva (mucus secreting epithelial membrane); tears (which contain lysozyme)
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Chemical barriers Fibronectin- high MW glycoprotein that binds to certain bacteria and aids in their clearance; can block attachment Beta-lysin- cationic (positively charged) polypeptide from platelets; can kills some Gram pos bacteria Interferons- low MW glycoprotein “cytokines” stimulated by viral infection; host species specific; virus nonspecific Bacteriocins- substances produced by some bacteria that kill other bacteria Complement System A complex of 20 plasma proteins (C’) Some act as an enzyme cascade- product of one reaction catalyzes the next Must be cleaved into biologically active fragments Larger “b” fragments bind to cell surfaces Smaller “a” fragments stay in the plasma 1. C5a- inflammatory mediator- acts directly on vascular endothelial cells-vasodilation/permeablility 2. C3a- another inflammatory mediator 3. C3b- most abundant complement component; involved with opsonization Complement system is activated by two mechanisms 1. Classical pathway involves specific antibody; complex interaction of C’ components; the membrane attack complex (MAC) is composed of C5b6789; cells are lysed 2. Alternative pathway involves another plasma protein, properdin; ultimately MAC is formed and cell lysis occurs
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Normal microbiota (normal flora) (indigenous microbiota)
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326Minnimm - Host Barriers General barriers...

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