There is no single cause that explains all autoimmune diseases. Current evidence indicates that most arise through an interaction between genetic susceptibility and environmental or biological triggers, with additional influences from age, sex, hormones, infections, medications, smoking, metabolic factors and other exposures depending on the disease. Having a genetic risk factor does not mean that autoimmune disease will inevitably develop; many people who carry susceptibility variants never become ill. Conversely, people without a known family history can develop autoimmune disease. Researchers therefore generally regard autoimmune disease as multifactorial, meaning that several factors can combine to disturb immune tolerance and initiate or sustain an abnormal immune response.
Genetics contributes by influencing how immune cells recognise antigens, regulate activation and maintain tolerance. Human leukocyte antigen, or HLA, variants are particularly important in susceptibility to numerous autoimmune conditions because HLA molecules present peptide fragments to T cells and thereby influence which immune responses can occur. Other genetic variants affect cytokine signalling, lymphocyte activation, regulatory pathways and mechanisms that remove damaged or dying cells. Genetic risk is usually polygenic rather than controlled by one gene, although rare single-gene disorders can produce strong autoimmune phenotypes. A family history therefore increases risk for some diseases but does not provide a simple inheritance pattern comparable with a single-gene genetic disorder.
Environmental exposures can provide triggers in genetically susceptible people. Infectious agents have received particular attention because infection can stimulate strong immune responses, alter tissue environments or produce molecular structures that resemble self-antigens. Epstein-Barr virus, for example, has been implicated in research into multiple sclerosis and other autoimmune phenomena, although identifying an association or trigger does not mean that the virus alone causes every case. Other environmental influences include ultraviolet radiation, smoking, occupational exposures, certain chemicals and dietary or respiratory exposures, depending on the disease. Researchers continue to investigate how such exposures interact with genetically determined immune regulation rather than assuming that any one exposure is universally responsible.
One proposed mechanism is molecular mimicry, in which immune responses generated against a microbial antigen may cross-react with structurally similar molecules in the host. Other mechanisms include bystander activation, where inflammatory signalling activates autoreactive cells that were previously controlled; epitope spreading, where tissue damage exposes additional self-antigens and broadens the immune response; and defective clearance of apoptotic cellular material, which can increase exposure to intracellular self-antigens. Loss of regulatory T-cell control and abnormalities in B-cell tolerance can also permit autoreactive lymphocytes to persist or become activated. These mechanisms are not mutually exclusive, and different autoimmune diseases may use different combinations of them.
Hormonal and sex-related factors are relevant because many autoimmune diseases are more common in women than men, although the magnitude and explanation vary by disease. Sex hormones can influence immune-cell development, cytokine signalling, antibody production and tolerance, while genetic differences between sex chromosomes may also affect immune regulation. Pregnancy can temporarily alter immune activity and disease patterns, and autoimmune conditions may improve or worsen during pregnancy or after delivery depending on the disorder. Age is another determinant because immune regulation changes across the lifespan. These observations demonstrate that autoimmune susceptibility is influenced by biological context rather than being determined exclusively by inherited DNA sequence.
Lifestyle and metabolic factors can modify risk or disease activity without being universal causes. Smoking is a recognised risk factor for some autoimmune diseases, particularly rheumatoid arthritis, while obesity can influence inflammatory signalling and disease burden in several conditions. Sun exposure can trigger disease activity in certain genetically susceptible individuals, as described for some forms of myositis and lupus-related disease. Some medications can produce drug-induced autoimmune syndromes or alter immune regulation, but these effects are specific to particular drugs and susceptible individuals. It is therefore inaccurate to claim that a single lifestyle choice, food, psychological event or environmental exposure causes autoimmune disease in general.
Autoimmune disease is also influenced by failures in immune tolerance, which is the collection of mechanisms preventing immune responses against healthy self-tissues. Central tolerance removes or modifies many strongly self-reactive lymphocytes during development in the thymus and bone marrow, while peripheral tolerance controls autoreactive cells that escape into the circulation. Regulatory T cells, inhibitory receptors, anergy, deletion and other mechanisms help maintain this control. Autoimmunity can develop when genetic susceptibility, tissue injury, infection or other signals overcome these safeguards. Once self-reactive immune cells become activated, inflammatory feedback can perpetuate the response, creating a distinction between the initial trigger and the mechanisms responsible for chronic disease.
Researchers therefore describe autoimmune causation probabilistically rather than as a simple chain with one identifiable cause. The same genetic predisposition can produce different diseases in different individuals, and the same disease can arise through different combinations of triggers. Many proposed environmental associations remain under investigation, and discovering a risk factor does not establish that it is sufficient to cause disease.
Current research aims to identify which combinations of genetic susceptibility, microbial exposure, environmental factors and immune abnormalities produce specific autoimmune phenotypes, with the long-term goal of improving prediction, prevention and targeted treatment. For most autoimmune diseases, the precise initiating event remains unknown even though many downstream immune mechanisms are increasingly understood.
