Virtually the entire eukaryotic world shares the same immune defense system: innate immunity, based on the identification of molecular profiles specific to the microbial world and not present in the host. All cells have such a program, with receptors located on the cell surface, in the cytoplasm or in endosomal compartments. Their activation induces a limited number of signal transmission systems, which in turn rapidly (within a few hours) induce transcriptional activation of genes coding for a series of proteins with direct or indirect anti-infective action, such as interleukin 1 or interferons. The principle of this system is remarkably conserved from C. elegans, Drosophila, to man. It has diversified in the course of evolution. Mammals have five classes of receptors. The molecular aspects of receptor recognition and signaling have been identified in recent years. In parallel, human medical genetics (effects of mutations) and evolutionary genetics point to the non-redundant nature of some of these receptors, and identify essential functions in anti-bacterial, fungal or viral immunity.
To a certain extent, the "price to pay" for the evolutionary adjustment of innate immunity to human host microorganisms - better resistance - is an increased risk of inflammatory or autoimmune disease, linked to a higher intensity response.
In addition to recognized microbial molecular profiles, receptors can in fact recognize products from cells under stress, thereby increasing the reactivity of innate immunity responses to external aggression. The ways in which innate immunity is activated (the types of receptors and effectors) dictate the response profile of adaptive immunity, which is induced secondarily. The integration of these signals takes place primarily at the level of antigen-presenting cells - the dendritic cells - as discussed in an earlier lecture. Innate immunity thus includes cells specialized in phagocytosis and microbicidia, cells recruited by the first signals emitted, as well as lymphocytes whose importance has very recently been recognized: innate lymphocyte cells.
