Basic concepts in immunology: defense from infectious disease. Microbes help us fight against pathogen: by simply taking up space, by stimulating the immune system, by exposing us so our body develops antibodies; decrease in microbes = immunity over reacts to small pathogens. Three levels of immunity: primary: normal flora and anatomical barriers. Organisms that colonize on the bodies surfaces that don"t cause disease. Includes skin; normal flora, sweat, gi tract; fatty acids, bile salts, low ph, nasopharynx and eye; mucus, saliva, sneezing, coughing, Selective cell wall damage: lysozyme cuts glucosamine-muramic acid linkage. Lysozyme is abundant in a number of secretions (mucus, saliva and tears) Specificity limited: one cell can recognize many pathogens. There are six continuousprocess of phagocytosis into six steps: chemotaxis; Chemotaxis is the movement ofa cell either towards a chemical stimulus (positive chemotaxis) or away from achemical stimulus (negative chemotaxis) Positive chemotaxis involvesthe use of pseudopods to crawl toward microorganisms at the site of aninfection.