Innate immunity and the biological relevance of the acquired immune response.

Abstract

Innate immunity has been considered to be a semi-obsolescent hold-over from invertebrate immunity that had been largely superseded by the acquired (adaptive) immune system of vertebrates. Recently, however, it has become apparent that innate immunity collaborates with acquired immunity to create an effective anti-microbial immune response. This realization may enhance our understanding of host–pathogen interactions and auto-immune disease, and facilitate the development of vaccines. This editorial reviews some of the findings that have led to this new perspective on innate immunity. The immune system of vertebrates has been conceptually divided into two parts, innate and acquired immunity. There are many distinguishing features of the two systems, but the essential difference is genetic. The recognition proteins of the former are encoded in the germline, having evolved in invertebrates for the purpose of host defence against infection. They have been selected for their ability to recognize micro-organisms that pose risks to the host, and to initiate appropriate defensive reactions. In short, innate immunity provides evolutionary information on the biological correlates of structures with which its receptors interact; that is, these structures are likely to be associated with infectious organisms. However, innate immunity has the drawbacks of being able to recognize only the relatively few microbial structures that are highly conserved, and of being unable to evolve as rapidly as do micro-organisms. The receptors of acquired immunity, which are the antigen receptors of T and B lymphocytes, have overcome these problems by not being encoded in the germline. Rather, they are products of somatically rearranging elements, the V, D and J segments of T-cell receptor and antibody genes. This remarkable capability, which occurred suddenly with the evolution of vertebrates approximately 400 million years ago, creates an enormous repertoire of antigen-binding structures. Yet, the somatic generation of their genes has the …