ABSTRACT
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The events leading to the development of immunity directed against pathogens are exceedingly complex. There are two distinct systems-innate and adaptive-that act in concert as well as separately in the development of immunity. The innate system provides a rst line of defense against a foreign substance. It is nonspeci c, rapid, lacks immunologic memory, and is usually of short duration. The adaptive system has exquisite speci city, is slower in development, exhibits immunological memory, and is long lasting.
