Innate Immunity: Introduction

The concept of ‘innate immunity’ embraces all sorts of measures that exclude, inhibit, or slow down infections with little specificity and without adaptation or generation of a protective memory. The mammalian innate immune defenses described in this article comprise the complement system, nonspecific phagocytic and cytolytic leukocytes (macrophages, monocytes, granulocytes, natural killer cells, and dendritic cells), and cytokines such as the antivirally active type I interferons. Since the type I interferon (IFN-α/β) system is our primary defense against viral infections, special attention will be paid to the virus-triggered induction of IFN transcription, the signaling activated by IFNs, and the antiviral factors expressed as a consequence.