[Mechanisms of the induction of autoimmunity]

The main function of the immune system is to protect the body by responding to invading microorganisms. Immunologic tolerance is the basic property of the immune system that provides for self/non-self discrimination so that the immune system can protect the host from external pathogens without reacting against itself. Central tolerance is achieved by the clonal deletion of self-reactive lymphocytes expressing receptors with high avidity for self. Autoreactive lymphocytes which escaped selection in the central lymphoid organs are present in the peripheral repertoire but but are kept under control by multiple diverse peripheral tolerance mechanisms acting either directly on the self-reactive T cell (T-cell intrinsic) or indirectly via additional cells (T-cell extrinsic). Intrinsic mechanisms include ignorance of autoantigens, anergy, phenotype skewing or activation-induced cell death of autoreactive T lymphocytes, while extrinsic mechanisms act through immature and/or tolerogenic dendritic cells as well as different types of regulatory cells. Autoimmune diseases are associated with humoral or cell-mediated immune reactions against one or more of the body's own constituents. Activation and clonal expansion of autoreactive lymhocytes is a crucial step in the pathogenesis of autoimmune diseases. They result from the complex interactions between genetic traits and environmental factors, among which infections are the most likely cause. Several basic mechanisms may be operating whereby an infectious agent actually induces apparent autoimmne reactivity including molecular mimicry, bystander activation, induction of costimulation, polyclonal activation, altered processing and expression of cryptic antigens. Although many questions regarding autotolerance and etiop athogenestis of autoimmunity have yet to be resolved, it is evident that multiple overlapping pathways are operative in establishing, maintaining and breaking autotolerance, as well as during the initiation, progression, and final effector phases of autoimmunity.