Organisms resist infections by establishing barriers and activating different classes of innate resistance and adaptive immunity. The tissue in which the infection occurs regulates the strength, quality, and type of the immune response for efficient pathogen eradication and tissue repair while limiting collateral tissue damage. However, some microbes have evolved evasion mechanisms that thwart these host responses, resulting in uncontrolled or chronic infection associated with pathologic damage. The innate immune system is mainly centered on the physical and chemical barriers to infection, in addition to different cell types known to recognize invading pathogens and activate antimicrobial immune responses. The immune system’s natural capacity to detect and destroy abnormal cells may prevent the development of many cancers. However, cancer cells are sometimes able to avoid detection and destruction by the immune system. The human innate immune system is the body's first line of defense against the foreign pathogen and is pivotal in determining an outcome of such a confrontation. The ability of the pathogen to invade the human host relies on its capacity to evade and circumvent host defense mechanisms.