The role of the immune system is simple yet challenging: to eliminate pathogenic agents. The immune system contains nonspecific and specific components; that is, some constituents act without precise recognition of the target, others have exquisite specificity. Regulation of the immune response and maintenance of tolerance to self are critical to the health of the animal. To properly evaluate or control immune function, knowledge of the basic components of the normal immune system is essential. This review covers the current knowledge of individual components of the immune system and how they interact to protect the host from infectious disease agents.
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