An autoimmune disease is a disorder in which the immune system mounts an inappropriate response against the body's own cells, tissues or organs. Under normal circumstances, immune tolerance prevents immune cells from treating healthy self-tissues as dangerous targets, while allowing the system to recognise and respond to pathogens and other threats. In autoimmune disease, this tolerance is disturbed, allowing autoreactive T cells, B cells, autoantibodies or other immune mechanisms to participate in tissue injury. Autoimmune diseases are not one single illness but a large group of disorders with different targets, mechanisms, patterns of inflammation and clinical manifestations. More than 80 autoimmune diseases are recognised, ranging from conditions affecting one principal tissue to systemic disorders involving several organs.
Autoimmunity and autoimmune disease are related but not identical concepts. Autoreactive immune cells or autoantibodies can sometimes be detected without a person developing clinically significant disease, meaning that evidence of autoimmunity does not necessarily equal an autoimmune disorder. An autoimmune disease occurs when inappropriate immune recognition contributes to pathological tissue damage or altered organ function.
Some diseases are organ-specific, such as type 1 diabetes, in which immune-mediated destruction targets pancreatic beta cells, whereas others are systemic, such as systemic lupus erythematosus, in which immune abnormalities can affect multiple organs. The distinction is important because the presence of an autoimmune marker must be interpreted in clinical context rather than treated as a diagnosis by itself.
Autoimmune disease can involve several immune mechanisms. B cells can produce autoantibodies that bind self-antigens and may directly interfere with physiological functions or activate other immune pathways. T cells can recognise self-derived antigens and promote inflammatory or cytotoxic responses. Immune complexes can deposit in tissues and activate complement, while innate immune cells can amplify inflammation through cytokines and other mediators. Some disorders involve a predominantly antibody-mediated mechanism, some are strongly T-cell-mediated, and many involve overlapping mechanisms. The resulting tissue damage can be chronic, episodic or progressive, depending on the disease and its target. This mechanistic diversity explains why autoimmune diseases cannot all be diagnosed or treated using one universal test or treatment.
The organs affected depend on the particular autoimmune disease. Rheumatoid arthritis primarily affects joints but can also produce extra-articular manifestations; type 1 diabetes targets insulin-producing pancreatic beta cells; multiple sclerosis involves immune-mediated injury to structures of the central nervous system; lupus can affect skin, joints, kidneys, blood cells, lungs, heart and nervous tissue; Sjögren's disease commonly targets moisture-producing glands; and vitiligo involves destruction of pigment-producing melanocytes. Other autoimmune disorders affect the thyroid, gastrointestinal tract, skin, muscles, blood vessels, liver or multiple organ systems. Symptoms therefore vary enormously and may include pain, swelling, weakness, fatigue, skin changes, neurological symptoms, abnormal blood counts or impaired organ function.
Diagnosis generally depends on the pattern of symptoms and examination findings combined with laboratory tests, imaging or tissue assessment when appropriate. Tests may include inflammatory markers, disease-specific autoantibodies, blood-cell measurements, organ-function tests and other investigations. No single blood test diagnoses all autoimmune disease, and some patients with autoimmune disease may have negative results for particular antibodies. Conversely, autoantibodies can sometimes occur in people who do not have the corresponding disease. Clinicians therefore interpret laboratory findings in relation to symptoms, disease patterns and alternative explanations. This is particularly important because many autoimmune diseases can resemble infections, allergies, endocrine disorders, mechanical conditions or other inflammatory diseases during early stages.
Autoimmune diseases can be chronic because the immune system may continue to recognise self-components after the initiating process has occurred. Some conditions follow relapsing-remitting patterns, with periods of increased disease activity followed by quieter intervals, while others produce more persistent inflammation or progressive tissue damage. Treatment therefore commonly aims to reduce inappropriate immune activity, control inflammation, prevent organ damage and maintain function rather than simply eliminate a temporary trigger.
Depending on the disease, treatment may involve immunomodulatory or immunosuppressive medicines, biologic therapies, replacement of hormones or other substances lost through organ damage, symptom-directed treatment and monitoring for complications. Many autoimmune diseases currently have no definitive cure, although disease control can often be substantially improved.
Autoimmune disease should be distinguished from immunodeficiency and from ordinary inflammation. Immunodeficiency means that one or more components of host defence are insufficient or dysfunctional, increasing vulnerability to infections, whereas autoimmune disease involves inappropriate immune recognition or attack against self. Inflammation is a normal component of many immune responses and is not inherently autoimmune; it becomes pathological when excessive, persistent, misdirected or otherwise damaging. Allergy is another distinct form of immune-mediated disease in which the immune system reacts inappropriately to ordinarily harmless external substances such as pollen, foods or insect venom. Some people can have more than one immune-mediated condition, but the biological mechanisms and diagnostic categories remain different.
Modern understanding views autoimmune disease as the result of failures or alterations in immune tolerance occurring within a complex interaction of genetic susceptibility, environmental exposures, immune regulation, hormones, infections and other biological factors. The diseases were historically difficult to classify because their symptoms often overlapped with unrelated disorders, but advances in immunology, autoantibody testing, genetics, imaging and molecular biology have established many distinct mechanisms. Research continues to identify disease-specific immune pathways and potential triggers, including microbial exposures and environmental factors. The central definition remains that autoimmune disease results from an immune response directed against self, but the precise molecular route from susceptibility to clinical disease differs substantially between conditions.
