IMMUNE TOLERANCE: How the Immune System Learns to Recognise “Self” and Prevent Autoimmunity
Immune tolerance refers to the mechanisms that prevent the immune system from attacking the body’s own tissues. Because lymphocytes develop enormous diversity in antigen receptors, many newly formed T and B cells are capable of recognising self-antigens. Without strict tolerance processes, these cells would trigger harmful immune responses, resulting in autoimmune disease. Tolerance develops through carefully controlled mechanisms that eliminate or silence self-reactive lymphocytes during their development and throughout life. These processes ensure a balanced immune system that can recognise pathogens while remaining non-reactive to healthy tissues.
What You Need to Know
Immune tolerance is the collection of mechanisms that allow the immune system to distinguish between self and non-self, preventing destructive immune responses against the body’s own tissues. This process is essential because lymphocytes generate their antigen receptors randomly, meaning self-reactive cells are an inevitable by-product of normal immune development. Without effective tolerance, immune defence would quickly turn into autoimmune disease.
Tolerance operates at two main levels. Central tolerance occurs during early lymphocyte development, in the thymus for T cells and in the bone marrow for B cells, where strongly self-reactive cells are removed before they enter circulation. Peripheral tolerance controls any self-reactive lymphocytes that escape central checkpoints, regulating their activity once they are present in tissues and lymphoid organs.
Several key mechanisms work together to maintain immune tolerance:
Deletion of strongly self-reactive lymphocytes during development
Failure of activation when antigen recognition occurs without costimulatory signals
Active suppression by regulatory immune cells that limit inappropriate responses
Peripheral tolerance relies heavily on regulatory T cells, often called Tregs, which suppress immune activation through direct cell contact and cytokine release. Other mechanisms include anergy, a state in which lymphocytes remain alive but functionally inactive, and immune ignorance, where self-antigens are physically sequestered from immune recognition. These overlapping layers allow the immune system to remain responsive to infection while avoiding damage to healthy tissues.
Beyond the Basics
Central Tolerance in the Thymus and Bone Marrow
Central tolerance is the first checkpoint in immune education and operates during early lymphocyte development. In the thymus, developing T cells are exposed to self-antigens presented on self-MHC molecules. Positive selection ensures that T cells can recognise self-MHC, a requirement for any meaningful immune response. Cells that fail to bind MHC at all undergo apoptosis because they would be unable to interact with antigen-presenting cells in the periphery.
Negative selection then removes T cells that bind too strongly to self-antigens. This step prevents highly self-reactive cells from entering circulation. Thymic epithelial cells play a critical role by presenting a wide range of self-proteins, including tissue-specific antigens that would otherwise never be encountered in the thymus. This broad exposure reduces the likelihood that dangerous autoreactive T cells survive selection.
A similar process occurs during B cell development in the bone marrow. Immature B cells are tested for self-reactivity by exposure to self-antigens. Cells that bind strongly may undergo receptor editing, a process in which the antigen receptor is rearranged to reduce self-reactivity. If editing fails, the cell undergoes apoptosis. This early screening limits the number of autoreactive B cells that enter the peripheral immune system.
Peripheral Tolerance Mechanisms
Despite strict central selection, some self-reactive lymphocytes escape into circulation. Peripheral tolerance mechanisms exist to control these cells and prevent tissue damage once they are present in lymphoid organs and tissues.
One important mechanism is anergy, a state in which lymphocytes remain alive but functionally inactive. This occurs when a T cell recognises antigen in the absence of appropriate costimulatory signals, signalling that the antigen is not associated with danger. Anergic cells do not proliferate or produce effector cytokines, limiting the risk of autoimmune activation.
Regulatory T cells, often referred to as Tregs, provide active suppression of immune responses. These cells inhibit activation of other lymphocytes through direct cell contact and secretion of anti-inflammatory cytokines such as interleukin-10 and transforming growth factor beta. Peripheral deletion also contributes to tolerance, as repeated or persistent antigen stimulation can trigger apoptosis in self-reactive lymphocytes, preventing long-term immune activation against self-tissues.
Tolerance of Non-Harmful External Antigens
Immune tolerance extends beyond recognition of self. The immune system must also avoid unnecessary responses to harmless external antigens such as food proteins and commensal microbes. This is particularly important at mucosal surfaces, where antigen exposure is continuous.
In the gastrointestinal tract, dendritic cells present antigens in a context that favours tolerance rather than activation. This presentation often promotes differentiation of regulatory T cells rather than effector T cells. The result is immune restraint toward dietary antigens and the microbiome, preventing chronic inflammation in tissues that are constantly exposed to non-threatening material.
Breakdown of Tolerance and Autoimmunity
Failure of tolerance mechanisms allows self-reactive lymphocytes to become activated and cause tissue damage. This breakdown can occur due to genetic susceptibility, environmental triggers such as infection, or impaired regulatory cell function. Once tolerance is lost, immune responses against self-antigens can become self-sustaining.
Autoimmune diseases such as type 1 diabetes, systemic lupus erythematosus, and multiple sclerosis arise from inappropriate activation of autoreactive T cells, B cells, or both. Certain HLA types increase disease risk by presenting self-antigens in a way that favours immune activation rather than suppression. Understanding how tolerance is established and maintained helps explain why autoimmunity develops and why it targets specific tissues in different individuals.
Clinical Connections
Central and peripheral tolerance determine whether immune responses remain protective or become pathological. Failure of central tolerance allows highly self-reactive lymphocytes to enter circulation, while breakdown of peripheral tolerance permits these cells to become activated in tissues. Together, these failures lead to sustained immune attack against self-antigens and the development of autoimmune disease. Modern treatment strategies aim to limit this inappropriate activation rather than broadly suppress all immune function.
Several clinical applications arise directly from tolerance mechanisms:
Autoimmune disease management through suppression of autoreactive lymphocytes
Antigen-specific desensitisation to reduce immune responses to defined targets
Expansion or enhancement of regulatory T cell activity to limit tissue damage
Selective immunosuppression that preserves host defence while reducing autoimmunity
Therapies designed to promote tolerance include regulatory T cell based approaches, antigen-specific immunotherapy, and targeted immunosuppressive drugs that interrupt costimulatory signalling or cytokine pathways. These strategies aim to restrain self-reactive immune cells while maintaining the ability to respond to infection. Broad immunosuppression is increasingly avoided when more specific tolerance-promoting options are available, as it carries a higher risk of infection and malignancy.
Tolerance mechanisms are also central to transplantation medicine. Donor organs express foreign MHC molecules that activate recipient T cells unless immune responses are controlled. Immunosuppressive regimens are used to reduce T cell activation and promote a state of acquired tolerance, allowing graft survival while limiting rejection. Long-term transplant outcomes depend on maintaining this balance without completely disabling immune surveillance.
Variation in tolerance mechanisms also contributes to individual differences in immune disease. Some individuals develop allergies or autoimmune conditions following common environmental exposures, while others remain unaffected. Differences in regulatory cell function, antigen presentation, and costimulatory signalling influence whether immune activation or immune restraint dominates.
Concept Check
What is the main purpose of immune tolerance, and why is it essential for health?
How do central tolerance mechanisms remove self-reactive T and B cells during development?
What roles do anergy and regulatory T cells play in peripheral tolerance?
Why is mucosal tolerance important for preventing unnecessary immune responses?
How can failure of immune tolerance lead to autoimmune disease?