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Immunological Tolerance

For medical students2 min readUpdated 2026-10-10

Immunological tolerance is a specific state of unresponsiveness of the immune system to a particular antigen. This mechanism is critical for preventing damage to the body's own tissues.

Main functionPrevention of damage to the body's own tissues
System failureLoss of tolerance leads to the development of autoimmune diseases
Food tolerancePrevents hypersensitivity reactions to dietary proteins
Primary mechanismsClonal deletion and clonal anergy

Core Concept and Biological Significance

One of the primary functions of the human immune system is reliable defense against various pathogens. However, any powerful protective response must be strictly controlled. If this process is left unchecked, there is a high risk of severe tissue damage.

This is precisely why immunological tolerance exists. It is a state in which an immune response to a specific antigen fails to develop. The cornerstone of this phenomenon is the immune system's ability to accurately distinguish between "self" and "non-self." When this fine line is blurred and the ability to discriminate is impaired, the immune system begins to attack its own cells. Such a failure underlies the development of severe autoimmune diseases.

Acquired Tolerance

Tolerance is not only directed against self-tissues but can also be acquired during lifetime. Acquired tolerance is essential for preventing destructive hypersensitivity reactions upon initial and subsequent contact with environmental antigens.

A classic and most understandable example is the lack of an immune reaction to dietary proteins. Developing tolerance to them prevents undesirable and dangerous reactions during repeated food intake. Without this acquired property, normal nutrition would be impossible, as every meal would be treated by the body as a massive antigenic attack.

Mechanisms of Tolerance

To maintain a state of unresponsiveness to certain antigens, the immune system utilizes several complex pathways. Pathology and immunology distinguish three key mechanisms:

  1. Clonal deletion. This process involves the physical elimination of self-reactive (autoreactive) T cells and B cells. Cell clones potentially capable of attacking the body are simply removed.
  2. Clonal anergy. In this mechanism, lymphocytes persist but undergo prolonged or completely irreversible functional inactivation. This deep "sleep" is induced by specific contact with an antigen, after which the cell loses its ability to respond.
  3. Peripheral suppression. This is a mechanism of active regulation involving specialized regulatory T cells (Tregs). They purposefully inhibit autoreactivity by secreting specific molecules called cytokines. These cytokines effectively reduce the overall intensity of the developing immune response.

Autotolerance

Special attention in immunology is given to autotolerance—tolerance exclusively to the body's own antigens. It is important to understand the hierarchy of the defense mechanisms described above.

Among the three listed pathways, clonal deletion and clonal anergy act as the primary mechanisms of autotolerance. They provide baseline safety for the body's own tissues, ensuring that lymphocytes capable of triggering an autoimmune process are either eliminated or permanently switched off.

Mnemonic

How to easily remember the three mechanisms of tolerance? Use the acronym DAS: Deletion (absence of cells), Anergy (inactivation of cells), Suppression (cytokine-mediated inhibition).

Frequently asked questions

In which organs does clonal deletion of autoreactive T cells occur?

Clonal deletion of autoreactive T cells occurs in the central immune organ, the thymus. This process is a crucial mechanism of central tolerance (negative selection), during which autoaggressive lymphocyte clones are destroyed via apoptosis. The interaction of Fas receptors on self-reactive T cells with Fas ligands triggers their programmed cell death pathway.

Which specific cytokines are secreted by suppressor T cells to suppress the immune response?

Suppressor T cells (regulatory T cells) secrete suppressive cytokines such as IL-10 and TGF-$\beta$. The release of these substances reduces the intensity of the immune response and inhibits autoreactivity, ensuring peripheral tolerance to self-antigens. In addition, suppression of the T-cell response can also occur through direct contact-dependent inhibition, independent of cytokines.

What are the types of immunological tolerance according to their origin?

According to their origin, there are two main types of immunological tolerance:

  • Natural tolerance — formed during ontogeny, representing the physiological absence of an immune response to self-antigens of tissues and cells (autotolerance).
  • Acquired tolerance — necessary to prevent hypersensitivity reactions upon initial and subsequent contacts with foreign antigens (e.g., the absence of adverse reactions to dietary proteins).
What surface markers do suppressor T cells express?

Natural regulatory T cells (suppressor T cells) express $CD4^+$ and $CD25^{hi}$ (high expression of CD25) markers on their surface. In addition to surface receptors, they intracellularly contain the transcription repressor protein FOXP3, which is a key factor for their suppressive properties. The complete phenotype of such cells is designated as $Foxp3^+$ or $CD4^+CD25^+Foxp3^+$.

What happens if the body loses the ability to distinguish 'self' from 'non-self'?

Impairment of this crucial recognition ability underlies the development of autoimmune diseases, in which the immune system begins to destroy the body's own tissues.

What is the purpose of acquired tolerance?

It is necessary to prevent hypersensitivity reactions upon initial contact with an antigen. For example, the absence of a reaction to dietary proteins protects us during repeated meals.

Which mechanisms of autotolerance are considered primary?

The primary (baseline) mechanisms of tolerance to self-antigens include clonal deletion and clonal anergy.

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