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IMMUNE COMPLEX: Clusters formed when antigens and antibodies bind together.

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 not ordinarily develop.

IMMUNE RESPONSE: The activity of the immune system against foreign substances.

IMMUNE SYSTEM: The complex functions of the body that recognize foreign agents or substances, neutralize them and recall the response later when confronted with the same challenge.

IMMUNE THROMBOCYTOPENIC PURPURA (ITP): Also Idiopathic Immune Thrombocytopenic Purpura. A condition in which the body produces antibodies against the platelets in the blood, which are cells responsible for blood clotting. ITP is very common in HIV-infected people. See also Antibodies; Platelets.

IMMUNITY: A natural or acquired resistance to a specific disease. Im-munity may be partial or complete, long-lasting or temporary.

IMMUNODEFICIENCY: A deficiency of immune response or a disorder characterized by deficient immune response; classified as antibody (B cell), cellular (T cell), combined deficiency or phagocytic dysfunction disorders.

IMMUNOGEN: A substance, also called an antigen, capable of provoking an immune response. See also Antigen.

IMMUNOGENICITY: The ability of an antigen or vaccine to stimulate an immune response. See also Antigen.

IMMUNOGLOBULIN: See Antibodies.

IMMUNOLOGICAL SURVEILLANCE or IMMUNOSURVEILLANCE: See Surveillance.

IMMUNOMODULATOR: Any substance that influences the immune system.

IMMUNOSTIMULANT: Any agent or substance that triggers or enhances the body's defense; also called immunopotentiators.

IMMUNOSUPPRESSION: A state of the body in which the immune system is damaged and does not perform its normal functions. Immunosuppression may be induced by drugs or result from certain disease processes, such as HIV infection. See also Immune System.

IMMUNOTHERAPY: Treatment aimed at reconstituting an impaired immune system. See also Immune System.

IMMUNOTOXIN: A plant or animal toxin (i.e., poison) that is attached to a monoclonal antibody and used to destroy a specific target cell. See also Antibiotic; Monoclonal Antibody.

INCIDENCE: The number of new cases occurring in a given population over a certain period of time.

INCLUSION/EXCLUSION CRITERIA: The medical or social standards determining whether a person may or may not be allowed to enter a clinical trial. For example, some trials may not allow people with chronic liver disease or with certain drug allergies; others may exclude men or women, or only include people with a lowered T-cell count.

INCUBATION PERIOD: The time interval between the initial exposure to infection and appearance of the first symptom or sign of disease.

IND: See Investigational New Drug.

INFECTION: The state or condition in which the body (or part of the body) is invaded by an infectious agent (e.g., a bacterium, fungus or virus), which multiplies and produces an injurious effect (active infection). As related to HIV: Infection typically begins when HIV encounters a CD4+ cell. The HIV surface protein gp120 binds tightly to the CD4 molecule on the cell's surface. The membranes of the virus and the cell fuse, a process governed by gp41, another surface protein. The viral core, containing HIV's RNA, proteins and enzymes, is released into the cell. See CD4 (T4) or CD4+ Cells; gp41; gp120.

INFECTIOUS: Capable of being transmitted by infection, with or without actual contact. See also Infection.

INFORMED CONSENT: Type of protection available to people considering entering a drug trial. Before entering the trial, participants must sign a consent form that contains an explanation of: (a) why the research is being done, (b) what researchers want to accomplish, (c) what will be done during the trial and for how long, (d) what risks are in the trial, (e) what benefits can be expected from the trial, (f) what other treatments are available, and (g) the participant's right to leave the trial at any time. See also Clinical Trial.

INOCULATION: The introduction of a substance (inoculum; e.g., a vaccine, serum or virus) into the body to produce or to increase immunity to the disease or condition associated with the substance. See also Vaccine.

INSTITUTIONAL REVIEW BOARD (IRB): 1. A committee of physicians, statisticians, community advocates and others that ensures that a clinical trial is ethical and that the rights of study participants are protected. All clinical trials in the United States must be approved by an IRB before they begin. See also Clinical Trial. 2. Every institution that conducts or supports biomedical or behavioral research involving human subjects must, by federal regulation, have an IRB that initially approves and periodically reviews the research so as to protect the rights of human subjects.

INTEGRASE: An HIV enzyme used by the virus to integrate its genetic material into the host cell's DNA. See also DNA; Enzyme.

INTEGRATION: The process by which the different parts of an organism are made a functional and structural whole, especially through the activity of the nervous system and of hormones. As related to HIV: The process by which the viral DNA migrates to the cell's nucleus, where it is spliced into the host's DNA with the help of viral integrase. Once incorporated, HIV DNA is called the provirus and is duplicated together with the cell's genes every time the cell divides. Recent reports suggest that HIV's DNA also can integrate into the DNA of nondividing cells such as macro-phages and brain and nerve cells. See also Integrase; Macrophage.

INTENT TO TREAT: Analysis of clinical trial results that includes all data from patients in the groups to which they were randomized (i.e., assigned through random distribution) even if they never received the treatment. See also Clinical Trial.

INTERFERON: A general term used to describe a family of 20-25 proteins that cause a cell to become resistant to a wide variety of viruses. They are produced by cells infected by almost any virus.

INTERLEUKIN-2 (IL-2): One of a family of molecules that control the growth and function of many types of lymphocytes. Interleukin-2 is an immune system protein produced in the body by T cells. It has potent effects on the proliferation, differentiation and activity of a number of immune system cells, including T cells, B cells and natural killer cells. Commercially, IL-2 is produced by recombinant DNA technology and is approved by the Food and Drug Administration for the treatment of metastatic renal (i.e., kidney) cell cancer. Studies have shown that in the test tube, addition of IL-2 can improve some of the immunologic functions that are abnormal in HIV-infected patients. In addition, IL-2 is a growth factor for T cells, causing them to increase in number. In a clinical study with IL-2, it was found that in a small number of HIV-infected patients, IL-2 boosted levels of CD4+ T cells (i.e., the infection-fighting white blood cells normally destroyed during HIV infection) for more than two years, a far longer time than typically seen with currently available anti-HIV drugs. See also Biotechnology; B Lymphocytes; Genetic Engineering; Killer T Cells; Lymphocyte; T Cells.

INTRAVENOUS (IV): Of or pertaining to the inside of a vein, as of a thrombus, or an injection, infusion or catheter.

INVESTIGATIONAL NEW DRUG (IND): The status of an experimental drug after the Food and Drug Administration agrees that it can be tested in people.

IN VITRO: ("In glass"). An artificial environment created outside a living organism (e.g., a test tube or culture plate) used in experimental research to study a disease or process.

IN VIVO: ("In life"). Studies conducted within a living organism (e.g., animal or human studies).

IRB: SEE INSTITUTIONAL REVIEW BOARD