Immunity (i-mū′ni-tē) is the ability to resist damage from patho-gens, such as microorganisms; harmful chemicals, such as toxins released by microorganisms; and internal threats, such as cancer cells. Immunity is categorized as innate (i′ nāt, i-nāt′ ) immunity (also called nonspecific resistance) and adaptive immunity (also called specific immunity), although the two systems are fully integratedin the body. In innate immunity, the body recognizes and destroys certain pathogens, but the response to them is the same each time the body is exposed. In adaptive immunity, the body recognizes and destroys pathogens, but the response to them improves each time the pathogen is encountered.
Specificity and memory are characteristics of adaptive immu-nity, but not innate immunity. Specificity is the ability of adaptive immunity to recognize a particular substance. For example, innate immunity can act against bacteria in general, whereas adap-tive immunity can distinguish among various kinds of bacteria. Memory is the ability of adaptive immunity to “remember” pre-vious encounters with a particular substance. As a result, future responses are faster, stronger, and longer-lasting.
In innate immunity, each time the body is exposed to a sub-stance, the response is the same because specificity and memory of previous encounters are not present. For example, each time a bacterial cell is introduced into the body, it is phagocytized with the same speed and efficiency. In adaptive immunity, the response during the second exposure to the same bacteria is fasterand stronger than the response to the first exposure because the immune system exhibits memory for the bacteria from the first exposure. For example, following the first exposure to the bac-teria, the body can take many days to destroy them. During this time, the bacteria damage tissues, producing the symptoms of disease. Following the second exposure to the same bacteria, the response is rapid and effective. Bacteria are destroyed before any symptoms develop, and the person is said to be immune.
Innate and adaptive immunity are intimately linked. Most important, mediators of innate immunity are required for the initiation and regulation of the adaptive response.