Innate immunity.

For more than half a century immunological research has been almost exclusively orientated towards the acquired immune response and the mechanisms of immune tolerance. Major discoveries have enabled us to better understand the functioning of the specific immune system: the structure of antibody molecules, the genetic mechanisms leading to the molecular diversity of B (BCR) and T (TCR) lymphocyte antigen receptors, the biological function of major histocompatibility complex (MHC) molecules in the presentation of peptides to alpha/beta receptor bearing T lymphocytes, the processes of positive and negative selection of lymphocytes during the course of their differentiation. The major role of specific or acquired immunity has been shown by the rapidly lethal character of severe combined immune deficiency diseases and various alterations in the mechanisms of tolerance have been proposed to explain the chronic inflammatory illnesses which are considered to be auto-immune. Natural or innate immunity has been known since the first description of an inflammatory reaction attributed to Cornelius Celsus. It entered into the scientific era at the end of the 19th century with the discovery of phagocytes by Metchnikoff and of the properties of the complement system by Bordet [1] but due to the vastness of the field and its lack of clear definition, it failed to excite the interest of researchers. The discovery of cytokines and progress in knowledge of the mechanisms of the inflammatory reaction have certainly helped to banish preconceived ideas about natural immunity, which was wrongly labelled as non-specific. This has led to the proposition of a wider role for immune functions beyond the level of the cell or the organism [2] and to a better understanding of the importance of the immediate defence mechanisms and their role in the later orientation of the acquired response.