Immune tolerance: A state of unresponsiveness to a specific antigen or group of antigens to which a person is normally responsive. Immune tolerance is achieved under conditions that suppress the immune reaction and is not just the absence of a immune response.

Immune tolerance can result from a number of causes including:

• Prior contact with the same antigen in fetal life or in the newborn period when the immune system is not yet mature;
• Prior contact with the antigen in extremely high or low doses;
• Exposure to radiation, chemotherapy drugs, or other agents that impair the immune system;
• Heritable diseases of the immune system;
• Acquired diseases of the immune system such as HIV/AIDS.

Immune tolerance can be defined as a state in which a T cell can no longer respond to antigen. The T cell "tolerates" the antigen.