Autoimmunity occurs when the immune system mounts an inappropriate response against the body's own antigens. Under normal circumstances, immune tolerance prevents destructive responses against most self-components, but tolerance can fail at central or peripheral levels. Autoimmunity can produce antibodies directed against self-antigens, autoreactive T-cell responses, or both. Some autoimmune responses are transient or clinically insignificant, whereas persistent autoreactivity can cause chronic tissue injury and disease. Examples of autoimmune diseases include type 1 diabetes, rheumatoid arthritis, systemic lupus erythematosus, multiple sclerosis, autoimmune thyroid disease, and myasthenia gravis. Autoimmunity is not synonymous with autoimmune disease: measurable autoreactive immune responses can occur without sufficient tissue damage or clinical symptoms to constitute disease.
Central tolerance develops primarily in the thymus for T cells and in the bone marrow for B cells, where developing lymphocytes that strongly recognise self may be deleted, edited, or otherwise controlled. Because not every self-reactive lymphocyte can be eliminated, peripheral tolerance provides additional safeguards. Regulatory T cells, inhibitory receptors, anergy, immune privilege, and mechanisms controlling antigen presentation all help prevent inappropriate activation. Autoimmune disease can arise when these safeguards fail or become overwhelmed. Genetic predisposition can influence antigen presentation, lymphocyte signalling, regulatory mechanisms, and other components of immune tolerance. Environmental triggers such as infections, tissue injury, hormonal influences, smoking, ultraviolet exposure, or particular medications can contribute in susceptible individuals, although the precise combination varies considerably among diseases.
Autoimmune disorders can be organ-specific or systemic. Organ-specific autoimmune diseases predominantly affect one tissue or organ, as in type 1 diabetes affecting pancreatic beta cells or autoimmune thyroid diseases affecting thyroid tissue. Systemic autoimmune diseases can involve multiple organs because the targeted antigens or immune mechanisms are distributed throughout the body. Rheumatoid arthritis primarily affects joints but can produce systemic manifestations, while systemic lupus erythematosus can involve skin, joints, kidneys, blood cells, nervous tissue, and other organs. Some autoimmune conditions are mediated predominantly by antibodies, others by T cells, and many involve complex interactions between cellular and antibody-mediated immunity. Classification is therefore useful but does not imply that one immune mechanism operates in isolation.
Autoantibodies are antibodies directed against self-antigens and can have several effects. Some bind directly to cellular structures and promote tissue damage through complement activation, phagocytosis, or other mechanisms. Others alter receptor function without destroying the target cell, as occurs when antibodies stimulate or block particular receptors. T cells can directly attack target cells or release cytokines that recruit and activate other immune cells. Immune complexes formed by autoantibodies and self-antigens can deposit in tissues and activate inflammatory pathways. These mechanisms explain why autoimmune disease can involve destruction, inflammation, altered cellular signalling, vascular injury, or combinations of these processes.
Diagnosis of autoimmune disease is usually based on the combination of clinical features, laboratory findings, imaging, and sometimes tissue biopsy rather than on a single test. Autoantibody tests can support diagnosis but are not necessarily specific or diagnostic by themselves. Antinuclear antibodies, rheumatoid factor, anti-citrullinated protein antibodies, thyroid antibodies, and numerous disease-specific antibodies are examples of tests used in particular clinical contexts. Some healthy people have detectable autoantibodies, and positive tests can occur in infections or other conditions. Conversely, not every person with an autoimmune disease has a detectable disease-associated antibody. Test interpretation must therefore consider pre-test probability, symptoms, disease pattern, and the performance characteristics of the assay.
Treatment aims to control abnormal immune activity, prevent tissue damage, manage symptoms, and preserve organ function. Approaches include corticosteroids, conventional immunosuppressive medicines, biological therapies targeting specific immune molecules or cells, and other targeted treatments. Selection depends on the disease, organ involvement, severity, age, pregnancy considerations, infection risk, and previous treatment response. Immunosuppression can increase susceptibility to infection and sometimes other complications, so treatment requires monitoring. Some autoimmune diseases can enter remission, while others require long-term management. Treating associated conditions and monitoring for organ-specific complications can be as important as controlling the immune response itself.
The causes of Autoimmunity are multifactorial. Genetic susceptibility can be substantial, but inheritance is generally complex rather than determined by one gene. Human leukocyte antigen variants influence susceptibility to several autoimmune diseases because they affect antigen presentation to T cells. Sex hormones, age, infections, environmental exposures, microbiome composition, and stochastic immune events may also influence risk. Molecular mimicry is one proposed mechanism in which immune responses to microbial antigens cross-react with structurally similar self-antigens, although its importance differs between diseases. Epitope spreading can occur when tissue injury exposes additional self-antigens, potentially broadening an immune response over time. These mechanisms illustrate why autoimmune disease usually cannot be attributed to a single cause.
Autoimmunity must be distinguished from allergy, immunodeficiency, and ordinary inflammation. Allergy involves inappropriate immune responses to generally harmless external substances, whereas autoimmunity targets self-antigens. Immunodeficiency involves inadequate immune function rather than excessive recognition of self. Autoimmune disease can produce inflammation, but inflammation itself does not establish autoimmunity. Historically, the concept developed as immunology demonstrated that immune responses could be directed against self, leading to major advances in understanding tolerance and immune regulation. A common misconception is that autoimmune disease means the immune system simply becomes "too strong"; the problem is more accurately described as inappropriate specificity or regulation, and some autoimmune diseases involve highly specific immune defects rather than generalized immune overactivity.
