Idiosyncrasy An abnormal sensitivity to some drug or other substance which is peculiar to the individual. IDLH Immediately Dangerous to Life and Health.  A NIOSH estimate for the maximum level of exposure from which a person could exit in 30 minutes without escape-impairing symptoms or irreversible health effects. Immunotoxicity Toxicity of the immune system.  It may take several forms: hypersensitivity (allergy and autoimmunity), immunodeficiency, and uncontrolled proliferation (leukemia and lymphoma). In Vitro Outside a living organism (e.g., in a test tube). In Vivo Occurring within a living organism. Interactions Refers to measures of effects of simultaneous exposure to two or more substances.  The four types of interactions are: additive, antagonistic, potentiation, or synergistic. Investigational New Drug Application (IND) Application submitted to the FDA by a pharmaceutical company requesting permission to undertake clinical investigations of a test drug in humans. Irritation Local tissue reaction without involvement of an immunologic mechanism.  It is a reversible inflammation. Isomer A structurally different form of a chemical having the same molecular formula.