The immune system is confronted with an endless array of potential pathogens and immunogens and it must make a decision regarding the nature of the response that is invoked. Robust immune-mediated inflammation is necessary to purge some life-threatening infections. In other conditions, immune responses must be tempered to reduce the risk of irreparable damage to tissues that possess a limited capacity for regeneration. The diversity of pathogens is remarkable and includes microorganisms that range in size from the microscopic picornaviruses to tapeworms that measure up to 35 feet in length. The immune system must adjust its response to take into consideration the nature of the pathogen and the organs that are affected. In some conditions, this amounts to a compromise in which immunemediated inflammation is restrained or diverted in a manner that inflicts minimal injury to host cells. In still other cases, the immune response is all but silenced. These immunological adjustments are the basis for regional immunity and immune-privileged sites.
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