Immune tolerance is a fundamental academic concept and research field in immunology that investigates the biological mechanisms by which the immune system selectively refrains from mounting harmful inflammatory or destructive responses against specific antigens. This state of non-reactivity is crucial for preventing autoimmunity against self-components and for enabling the immune system to coexist with innocuous foreign substances, such as commensal microbes or transplanted tissues. Key areas of study within this field include the mechanisms of central tolerance (deletion or inactivation of self-reactive lymphocytes during development) and peripheral tolerance (regulation of mature lymphocytes in secondary lymphoid organs and tissues through mechanisms like anergy, ignorance, or suppression). Understanding immune tolerance is essential for developing therapeutic strategies for autoimmune diseases, allergies, and transplant rejection, and for improving cancer immunotherapies.