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Autoimmunity results from loss of self-tolerance through genetic predisposition (HLA associations), environmental triggers (infections, molecular mimicry), and breakdown of regulatory mechanisms (Treg deficiency, Breg dysfunction). Autoimmune diseases range from organ-specific (Type 1 diabetes, rheumatoid arthritis) to systemic (lupus). Diagnosis relies on detecting pathogenic autoantibodies and autoreactive T cells.
From your study of immune tolerance, you know that the immune system actively prevents self-reactivity through multiple checkpoints: central tolerance (deleting self-reactive lymphocytes during development) and peripheral tolerance (mechanisms like regulatory T cells that suppress any self-reactive cells that escape). Autoimmunity occurs when these safeguards fail, allowing the adaptive immune system to mount a sustained attack against the body's own tissues. Understanding autoimmunity requires thinking about it as a multi-hit process — no single factor is usually sufficient; instead, genetic susceptibility, environmental triggers, and regulatory failure must converge.
The genetic foundation of autoimmune susceptibility is dominated by HLA (human leukocyte antigen) genes, which encode the MHC molecules that present peptides to T cells. Certain HLA alleles are strongly associated with specific autoimmune diseases — for example, HLA-B27 with ankylosing spondylitis and HLA-DR4 with rheumatoid arthritis. The logic is straightforward: if a particular MHC variant happens to bind self-peptides effectively and present them to T cells, it increases the probability that self-reactive T cells will be activated. But HLA associations are not deterministic — most people carrying a risk allele never develop disease. Non-HLA genetic factors also contribute, including polymorphisms in genes encoding cytokines, co-stimulatory molecules, and regulatory pathways (such as CTLA-4 and AIRE, which you encountered in the context of T cell regulation and thymic selection).
Environmental triggers convert genetic susceptibility into active disease. The most studied mechanism is molecular mimicry, in which a pathogen's proteins share structural similarity with self-proteins. During an infection, T cells and antibodies generated against the pathogen cross-react with the mimicked self-antigen, triggering an autoimmune response that persists after the infection clears. Rheumatic fever following streptococcal infection is a classic example — antibodies against streptococcal M protein cross-react with cardiac myosin. Other environmental triggers include tissue damage that releases normally sequestered self-antigens (the cryptic antigen hypothesis), chronic infection that creates a sustained inflammatory environment, and microbial disruption of regulatory T cell function.
The breakdown of peripheral tolerance is the final common pathway. Even healthy individuals harbor some self-reactive T and B cells that escaped central deletion — peripheral tolerance normally keeps these cells in check through mechanisms you studied previously: anergy (functional inactivation), suppression by regulatory T cells (Tregs), and deletion of chronically stimulated self-reactive cells. When Treg numbers or function decline — due to genetic defects, inflammatory signals that override suppression, or cytokine imbalances — self-reactive cells become activated. Autoimmune diseases are classified by their scope: organ-specific diseases like Type 1 diabetes (destruction of pancreatic β cells) and Hashimoto's thyroiditis (destruction of thyroid tissue) involve immune attack restricted to one tissue, while systemic diseases like systemic lupus erythematosus (SLE) involve widespread autoantibody production against ubiquitous antigens like DNA and nuclear proteins, causing multi-organ damage. In both cases, the fundamental problem is the same: the adaptive immune system's exquisite specificity, normally directed outward, has turned inward.