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Immune Deficiency: a breakdown or inability of certain parts of the immune system to function, thus making a person susceptible to certain diseases that they would have not contracted with a healthy immune system. Immune deficiencies may be temporary or permanent and be triggered by genetic mutation, therapy with immunesuppressive drugs (as during organ transplants) or an infection such as HIV.

Immunocompetent: refers to an immune system capable of developing a normal protective response when confronted with invading microbes or cancer.

Immunocompromised: refers to an immune system in which the response to infections and tumors is subnormal.

Immunoglobulin (class=SpellE>Ig): a general term for antibodies, which bind onto invading organisms, leading to their destruction. There are five classes: IgA, IgD, IgM, IgE, IgG.

Immunoglobulin A (IgA): an immunoglobulin found in body fluids such as tears and saliva and in the respiratory, reproductive, urinary and gastrointestinal tracts. IgA protects the body.s mucosal surfaces from infection.

Immunoglobulin G (IgG): the prominent type of immunoglobulin existing in the blood. Also called gamma globulin.

Immunomodulator: a drug such as IL-2 (see) that alters, suppresses or strengthens the body.s immune system.

Immunosuppression: weakening of the immune response that occurs with HIV infection as well as with some antiviral or anticancer treatments.

Immunotherapy: treatment aimed at reconstituting an impaired immune system. Examples of experimental immunotherapies for AIDS include passive hyperimmune therapy (PHT), IL-2 and therapeutic vaccines (see).

Indication: purpose for which a drug is prescribed. The FDA-approved indications appear on a printed insert included with the manufacturer.s drug packaging.

IND Abbreviation for Investigational New Drug.

Infusion: the process of administering a medication to an individual by slowly injecting a dilute solution of the compound into a vein. Infusions are often used when the digestive system does not absorb appreciable quantities of a drug that is also too bulky or too toxic to be given by quick injection.

Interim Analysis: an intermediary analysis of clinical trial data, performed at a point at which enough data have been gathered to derive preliminary, but not necessarily complete conclusions. Interim analyses are performed to see whether continuation of a clinical trial is warranted.

Interleukin: one of a large group of glycoproteins that act as cytokines (see). The interleukins are secreted by and affect many different cells in the immune system.

Intramuscular (IM) injected directly into a muscle.

Intraocular: injected into the eye.

Intrathecal: injected directly into the spinal canal.

Intravenous (IV) injected directly into a vein.

Intravitreal: injected into the eye.s vitreous humor (see) between the lens and the retina.

Intrinsic activity is the maximal stimulatory response induced by a compound in relation to that of a given reference compound (See also Partial agonist) This term has evolved with common usage. It was introduced by Ariëns as a proportionality factor between tissue response and receptor occupancy. The numerical value of intrinsic activity (alpha) could range from unity (for full agonists, i.e., agonist inducing the tissue maximal response) to zero (for antagonists), the fractional values within this range denoting partial agonists. Ariëns. original definition equates the molecular nature of alpha to maximal response only when response is a linear function of receptor occupancy. This function has been verified. Thus, intrinsic activity, which is a drug and tissue parameter, cannot be used as a characteristic drug parameter for classification of drugs or drug receptors. For this purpose, a proportionality factor derived by null methods, namely, relative efficacy, should be used. Finally, .intrinsic activity. should not be used instead of .intrinsic efficacy.. A .partial agonist. should be termed .agonist with intermediate intrinsic efficacy. in a given tissue.

Institutional Review Board (IRB): a regulating committee composed of internal staff, hospital affiliates and community members which reviews and approves all human trials conducted within a particular hospital or research center. The IRB ensures that a trial is conducted in an ethical manner, with proper protection of human subjects.

Inverse agonist is a drug which acts at the same receptor as that of an agonist, yet produces an opposite effect. Also called negative antagonists.

Investigational New Drug (IND) status given an experimental drug after the FDA approves an application for testing it in people.

Isosteres are molecules or ions of similar size containing the same number of atoms and valence electrons, e.g., O^2-, F^-, Ne (See also Bioisostere).