Clinical meaning
Autoimmune pathogenesis begins with the breakdown of immunological self-tolerance, a multilayered system that normally prevents the immune system from attacking host tissues. Central tolerance occurs during lymphocyte development: in the thymus, developing T cells that strongly recognize self-antigens presented by medullary thymic epithelial cells (via the AIRE gene, which drives ectopic expression of tissue-specific antigens) undergo apoptosis through negative selection. In the bone marrow, B cells that strongly bind self-antigens undergo receptor editing, anergy, or deletion. Peripheral tolerance provides additional safeguards through regulatory T cells (CD4+CD25+FoxP3+ Tregs) that actively suppress autoreactive effector T cells via IL-10 and TGF-beta secretion, T-cell anergy from antigen presentation without costimulatory signals (CD28-B7 interaction), and activation-induced cell death (AICD) via Fas-FasL. Molecular mimicry is a key trigger: microbial antigens that share structural homology with self-antigens activate cross-reactive T and B cells (e.g., Group A Streptococcus M protein mimics cardiac myosin, leading to rheumatic heart disease; Campylobacter jejuni lipooligosaccharides mimic gangliosides, triggering Guillain-Barre syndrome). Epitope spreading occurs when tissue damage from an initial immune response exposes previously hidden self-antigens (cryptic epitopes), generating new waves of autoreactive lymphocyte activation that perpetuate and diversify the autoimmune response. HLA molecules are the strongest genetic risk factors: specific HLA alleles preferentially present certain self-peptides to T cells, explaining disease-specific HLA associations.