Autoimmunity is the recognition of, and immune response against, the body’s own constituents by the immune system. These constituents, known as self-antigens, may include proteins and other cellular molecules. Autoimmunity can involve self-reactive lymphocytes or antibodies and does not invariably produce illness. An autoimmune disease develops when self-directed immune activity causes tissue injury or functional disturbance. The distinction between immune self-reactivity and clinically apparent disease is important: some autoantibodies occur in healthy people, whereas others are associated with particular disorders. (ncbi.nlm.nih.gov)
Self-recognition and immune tolerance
The immune system must respond to potentially harmful agents while limiting destructive responses against the host. Adaptive immunity depends on lymphocytes with diverse antigen receptors; some of these receptors can recognize self. Immune tolerance comprises mechanisms that remove, inactivate, or control such cells. Autoimmunity therefore cannot be understood simply as the presence of self-reactive cells: their activation, regulation, and effects on tissues also matter. (ncbi.nlm.nih.gov)
Central immune tolerance operates during lymphocyte development. Developing T cells undergo selection in the thymus, while developing B cells encounter tolerance mechanisms in the bone marrow. Strong recognition of self can lead to clonal deletion. B cells may also undergo receptor editing, changing their antigen-receptor specificity. The autoimmune regulator, commonly called AIRE, helps thymic cells express tissue-restricted antigens, allowing developing T cells to encounter constituents otherwise associated with distant organs. (ncbi.nlm.nih.gov)
Central tolerance is incomplete. Peripheral immune tolerance controls self-reactive lymphocytes that reach other tissues. Its mechanisms include anergy, a state of functional unresponsiveness, and suppression by regulatory T cells. Activation of T cells normally requires additional signals beyond antigen recognition, including costimulation. These regulatory requirements help prevent recognition of self from automatically becoming a damaging immune response. (ncbi.nlm.nih.gov)
Mechanisms of injury and dysfunction
Autoimmune responses use many of the same mechanisms as protective immunity, but their targets are host constituents. B cells can produce autoantibodies, which are antibodies directed against self-antigens. Some bind directly to structures in tissues; others form antigen–antibody complexes. T cells can contribute through direct cellular injury or by coordinating other immune responses. These mechanisms overlap, so a disease need not be exclusively antibody-mediated or T-cell-mediated. (ncbi.nlm.nih.gov)
Autoimmunity can disrupt function without simply destroying the targeted tissue. In Graves’ disease, thyroid-stimulating antibodies act on thyroid cells in a manner resembling thyroid-stimulating hormone, promoting excessive thyroid-hormone production. This illustrates why autoimmune disease includes inappropriate stimulation of normal physiological processes as well as tissue damage. (niddk.nih.gov)
Conversely, destruction of a specialized cell population can remove an essential function. In type 1 diabetes, the immune system destroys pancreatic beta cells, reducing production of insulin. Autoantibodies can help identify the autoimmune nature of diabetes, but their detection and the cellular processes responsible for beta-cell loss are distinct aspects of the disease. (niddk.nih.gov)
Susceptibility and initiating factors
Most autoimmune diseases arise through interactions between inherited susceptibility and environmental influences rather than a single universal cause. Relevant genetic differences include variants in the major histocompatibility complex, called the human leukocyte antigen system in humans. These molecules participate in presenting antigen fragments to T cells, linking genetic variation to differences in immune recognition. Genetic susceptibility increases risk but does not by itself establish that disease will develop. (niehs.nih.gov)
Environmental associations vary by disease. Tobacco smoke and certain occupational exposures have been associated with rheumatoid arthritis, while other exposures are investigated in different autoimmune conditions. An association with one disease should not be generalized to all forms of autoimmunity. Many initiating events remain unidentified, and evidence about exposures differs in strength and specificity. (niehs.nih.gov)
Infections can sometimes contribute through molecular mimicry: immune responses against microbial antigens cross-react with structurally similar host antigens. Infection-associated immune activation can also alter the conditions under which self-reactive cells become active. These are mechanisms applicable to particular settings, not evidence that every autoimmune disease has an infectious origin. (ncbi.nlm.nih.gov)
Clinical patterns
Autoimmune diseases are often described as organ-specific or systemic. Organ-specific conditions predominantly affect a particular organ or cell population, as in type 1 diabetes and autoimmune thyroid disease. Systemic conditions can involve several tissues and organs, although their manifestations differ substantially between individuals. These categories describe patterns of involvement rather than completely separate immune mechanisms. (ncbi.nlm.nih.gov)
Systemic lupus erythematosus can affect the skin, joints, kidneys, blood cells, and other organs. Rheumatoid arthritis principally affects joint linings but can also produce problems outside the joints. Thus, even a disease associated with a characteristic tissue may have broader effects. Autoimmune diseases differ in their targets, course, severity, and response to treatment. (niams.nih.gov)
Detection and clinical management
Clinical diagnosis relates immune findings to symptoms, examination, and evidence of organ dysfunction. Autoantibody tests provide useful information but are not interchangeable with a diagnosis. For example, antinuclear antibodies are common in lupus, yet a positive result also occurs in healthy individuals. Lupus evaluation may combine antibody testing, blood counts, kidney-function measurements, urine testing, and tissue biopsy; no single test establishes the diagnosis. (niams.nih.gov)
Management is disease-specific. In lupus, established treatments include anti-inflammatory drugs, corticosteroids, immunosuppressants, and biologic therapies that affect selected immune pathways. Treatment aims include limiting disease activity and preventing organ damage. In type 1 diabetes, insulin replaces a function lost through beta-cell destruction rather than directly eliminating the underlying autoimmune response. These contrasting approaches reflect the distinction between controlling immune activity and addressing its physiological consequences. (niams.nih.gov)