ABSTRACT
Entry of a pathogenic organism into a susceptible host is followed by invasion and colonization of tissues, circumvention of the host immune response, and injury or decreased function of host tissues. Microbial immunity consists of several factors. Natural and acquired immune mechanisms facilitate the body’s resistance against microorganisms. Microbes vary in the lymphocyte responsiveness and effector mechanisms they elicit. The skill with which pathogenic microorganisms resist the host’s immune defense mechanisms governs their survival and pathogenicity. Paradoxically, the host response to a pathogenic microorganism, rather than the microbe itself, may induce injury to host tissues. Factors that determine the outcome of man’s encounter with pathogenic microorganisms include the microbe’s virulence and the size of the infecting dose on the one hand and specific defense mechanisms of the host on the other.
