Immune tolerance, also known as immunological tolerance or immunotolerance, is the immune system's state of unresponsiveness to substances or tissues that would otherwise trigger an immune response. It arises from prior exposure to a specific antigen and contrasts the immune system's conventional role in eliminating foreign antigens. Depending on the site of induction, tolerance is categorized as either central tolerance, occurring in the thymus and bone marrow, or peripheral tolerance, taking place in other tissues and lymph nodes. Although the mechanisms establishing central and peripheral tolerance differ, their outcomes are analogous, ensuring immune system modulation.

Immune tolerance is important for normal physiology and homeostasis. Central tolerance is crucial for enabling the immune system to differentiate between self and non-self antigens, thereby preventing autoimmunity. Peripheral tolerance plays a significant role in preventing excessive immune reactions to environmental agents, including allergens and gut microbiota. Deficiencies in either central or peripheral tolerance mechanisms can lead to autoimmune diseases, with conditions such as systemic lupus erythematosus, rheumatoid arthritis, type 1 diabetes, autoimmune polyendocrine syndrome type 1 (APS-1), and immunodysregulation polyendocrinopathy enteropathy X-linked syndrome (IPEX) as examples. Furthermore, disruptions in immune tolerance are implicated in the development of asthma, atopy, and inflammatory bowel disease.

In the context of pregnancy, immune tolerance is vital for the gestation of genetically distinct offspring, as it moderates the alloimmune response sufficiently to prevent miscarriage.

However, immune tolerance is not without its drawbacks. It can permit the successful infection of a host by pathogenic microbes that manage to evade immune elimination. Additionally, the induction of peripheral tolerance within the local microenvironment is a strategy employed by many cancers to avoid detection and destruction by the host's immune system.

The phenomenon of immune tolerance was first described by Ray D. Owen in 1945, who noted that dizygotic twin cattle sharing a common placenta also shared a stable mixture of each other's red blood cells (though not necessarily 50/50), and retained that mixture throughout life. Although Owen did not use the term immune tolerance, his study showed the body could be tolerant of these foreign tissues. This observation was experimentally validated by Leslie Brent, Rupert E. Billingham and Peter Medawar in 1953, who showed by injecting foreign cells into fetal or neonatal mice, they could become accepting of future grafts from the same foreign donor. However, they were not thinking of the immunological consequences of their work at the time: as Medawar explains:^{[citation needed]}

We did not set out with the idea in mind of studying the immunological consequences of the phenomenon described by Owen; on the contrary, we had been goaded by Dr. H.P. Donald into trying to devise a foolproof method of distinguishing monozygotic from dizygotic twins....

However, these discoveries, and the host of allograft experiments and observations of twin chimerism they inspired, were seminal for the theories of immune tolerance formulated by Sir Frank McFarlane Burnet and Frank Fenner, who were the first to propose the deletion of self-reactive lymphocytes to establish tolerance, now termed clonal deletion. Burnet and Medawar were ultimately credited for "the discovery of acquired immune tolerance" and shared the Nobel Prize in Physiology or Medicine in 1960.

In their Nobel Lecture, Medawar and Burnet define immune tolerance as "a state of indifference or non-reactivity towards a substance that would normally be expected to excite an immunological response." Other more recent definitions have remained more or less the same. The 8th edition of Janeway's Immunobiology defines tolerance as "immunologically unresponsive...to another's tissues."