The host immune response is an integrated system of natural (innate) and specific immune mechanisms. By recognizing antigen with exquisite specificity, the specific immune responses direct and focus inflammatory processes to the site of antigen where they orchestrate and amplify the natural immune reactions. In contrast to the innate immune response, the specific immune system memorizes antigenic encounters. In subsequent encounters, as is the case with antigen reexposure or chronic antigen persistence, specific immune responses become increasingly effective and, therefore, more difficult to interrupt by immunosuppressive therapy. Like physiological immune responses, the inflammation in vasculitic syndromes has components of specific and of natural immunity. The spectrum varies from dominant T-cell involvement in large-vessel vasculitides to the predominance of innate immune mechanisms in small-vessel inflammation. Even within a vasculitic entity, the relative contribution of the different immune mechanisms can vary, possibly as a function of disease duration as exemplified in polyarteritis nodosa.
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