immunity
Active versus Passive Immunity Immunity to pathogens, and the ability to control pathogen growth so that damage to the tissues of the body is limited, can be acquired by 1 the active development of an immune response in the infected individual or 2 the passive transfer of immune components from an immune individual to a nonimmune one. Both active and passive immunity have examples in the natural world and as part of medicine. Active immunity is the resistance to pathogens acquired during an adaptive immune response within an individual Table 21.6 . Naturally acquired active immunity, the response to a pathogen, is the focus of this chapter. Artificially acquired active immunity involves the use of vaccines. A vaccine is a killed or weakened pathogen or its components that, when administered to a healthy individual, leads to the development of immunological memory a weakened primary immune response without causing much in the way of symptoms. Thus, with the use of vaccines, one can avoid the damage from disease that results from the first exposure to the pathogen, yet reap the benefits of protection from immunological memory. The advent of vaccines was one of the major medical advances of the twentieth century and led to the eradication of smallpox and the control of many infectious diseases, including polio, measles, and whooping cough.