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In basic terms, the immune system has two lines of defense: innate immunityand adaptive immunity. Innate immunity is the first immunological, nonspecific(antigen-independent) mechanism for fighting against an intrudingpathogen. It is a rapid immune response, occurring within minutes or hours
after aggression, that has no immunologic memory. Adaptive immunity, on the other hand, is antigen-dependent and antigen-specific; it has the capacity
for memory, which enables the host to mount a more rapid and efficient immune response upon subsequent exposure to the antigen. There is a
great deal of synergy between the adaptive immune system and its innate counterpart, and defects in either system can provoke illness or disease, such
as autoimmune diseases, immunodeficiency disorders and hypersensitivity reactions. This article provides a practical overview of innate and adaptive
immunity, and describes how these host defense mechanisms are involved in both health and illness.