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Immune Tolerance – Definition, Mechanisms & Importance

Immune tolerance is the ability of the immune system to recognize and accept the body´s own structures without attacking them, preventing autoimmune diseases.

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Things worth knowing about "Immune Tolerance"

Immune tolerance is the ability of the immune system to recognize and accept the body´s own structures without attacking them, preventing autoimmune diseases.

What Is Immune Tolerance?

Immune tolerance refers to the capacity of the immune system to recognize specific antigens – substances that can normally trigger an immune response – and deliberately refrain from attacking them. This is especially critical for the body´s own structures (self-antigens) but also applies to harmless environmental substances such as food components or pollen. When immune tolerance fails, the immune system may begin to target the body´s own tissues, leading to autoimmune diseases.

Types of Immune Tolerance

Central Tolerance

Central tolerance is established during the maturation of immune cells in the primary lymphoid organs, primarily the thymus (for T cells) and the bone marrow (for B cells). Immune cells that react strongly against the body´s own structures are identified and eliminated or permanently inactivated through a process called clonal deletion. This prevents self-reactive immune cells from entering circulation.

Peripheral Tolerance

Peripheral tolerance acts as a second line of defense within tissues and the bloodstream, targeting self-reactive immune cells that escaped central tolerance. Key mechanisms include:

Anergy: Immune cells are functionally inactivated when they encounter an antigen without sufficient co-stimulatory signals.
Regulatory T cells (Treg): Specialized T cells actively suppress immune responses against self-antigens.
Clonal deletion: Self-reactive cells in peripheral tissues can also be eliminated via programmed cell death (apoptosis).
Cytokine-mediated suppression: Signaling molecules such as IL-10 and TGF-beta dampen excessive immune reactions.

Importance of Immune Tolerance

Intact immune tolerance is essential for overall health. It allows the body to distinguish between harmful pathogens and harmless substances. It plays a particularly vital role in the following areas:

Pregnancy: The maternal immune system tolerates the genetically semi-foreign fetus without rejecting it.
Gut health: The intestinal immune system continuously maintains tolerance toward a vast array of food antigens and commensal bacteria.
Transplant medicine: After organ transplantation, the recipient´s immune system must be guided toward tolerating the donor organ.
Allergies: Impaired tolerance toward harmless environmental substances can lead to allergic reactions.

Breakdown of Immune Tolerance

When immune tolerance breaks down, the immune system may attack the body´s own structures. This results in autoimmune diseases such as:

Rheumatoid arthritis (attack on joint tissue)
Type 1 diabetes (destruction of insulin-producing cells in the pancreas)
Multiple sclerosis (attack on the myelin sheaths of nerve fibers)
Systemic lupus erythematosus (attack on various organs and tissues)

The exact causes of immune tolerance failure are complex and involve genetic predisposition, environmental triggers, infections, and hormonal factors.

Immune Tolerance in Clinical Medicine

Understanding immune tolerance has significant clinical implications. In allergology, allergen immunotherapy (desensitization) aims to induce tolerance toward specific allergens. In oncology, novel immune checkpoint inhibitors work by disrupting tumor-induced tolerance, allowing the immune system to attack cancer cells. In transplant medicine, immunosuppressive drugs are used to artificially establish tolerance toward donor organs.

References

Abbas AK, Lichtman AH, Pillai S. Cellular and Molecular Immunology. 10th ed. Elsevier; 2021.
Janeway CA Jr et al. Immunobiology: The Immune System in Health and Disease. 9th ed. Garland Science; 2016.
World Health Organization (WHO). Autoimmune diseases – Overview and current research directions. WHO Press; 2022.

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