I

IDIOPATHIC:
Without a known cause.

IDIOTYPES:
The unique and characteristic parts of an antibody's variable region, which can themselves serve as antigens.

IHS:
See Indian Health Service.

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 body's complicated natural defense against disruption caused by invading foreign agents (e.g., microbes, viruses). There are two aspects of the immune system's response to disease: innate and acquired. The innate part of the response is mobilized very quickly in response to infection and does not depend on recognizing specific proteins or antigens foreign to an individual's normal tissue. It includes complement, macrophages, dendritic cells, and granulocytes. The acquired, or learned, immune response arises when dendritic cells and macrophages present pieces of antigen to lymphocytes, which are genetically programmed to recognize very specific amino acid sequences. The ultimate result is the creation of cloned populations of antibody-producing B cells and cytotoxic T lymphocytes primed to respond to a unique pathogen.

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 persons infected with HIV.

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

IMMUNIZATION:
To protect against an infectious disease by vaccination, usually with a weakened (attenuated) or killed form of the disease-causing microorganism. While people are usually immunized against an infectious disease by getting vaccinated, having a disease such as measles, mumps, or rubella one time usually prevents or "immunizes" a person from getting this disease again.

IMMUNOCOMPETENT:
1. Capable of developing an immune response. 2. Possessing a normal immune system.

IMMUNOCOMPROMISED:
Refers to an immune system in which the ability to resist or fight off infections and tumors is subnormal.

IMMUNODEFICIENCY:
Breakdown in immunocompetence when certain parts of the immune system no longer function. This condition makes a person more susceptible to certain diseases.

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

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

IMMUNOGLOBULIN (Ig):
(Ig) Also called immune serum globulin. A class of proteins also known as antibodies made by the B cells of the immune system in response to a specific antigen, and containing a region that binds to this antigen (antigen-binding site). Each immunoglobulin is made up of two polypeptides, a heavy chain and a light chain. These heavy and light chains complex to form the antigen binding site. Each group of immunoglobulins is named for the type of heavy chain that it contains. The heavy chain determines the post-antigen binding activity of an antibody. There are five classes of immunoglobulins: IgA, IgD, IgE, IgG, and IgM.

IMMUNOGLOBULIN A (IgA):
(IgA) A class of antibodies, often formed as a dimer (i.e., two antibody molecules attached to each other end to end), that is secreted into bodily fluids such as saliva. IgA protects the body's mucosal surfaces from infections.

IMMUNOGLOBULIN D (IgD):
(IgD) A class of antibodies that is present in low concentration in serum. The primary function of IgD appears to be as an antigen receptor on mature B cells.

IMMUNOGLOBULIN E (IgE):
(IgE) A class of antibodies involved in anti-parasite (see parasite) immunity and in allergies.

IMMUNOGLOBULIN G (IgG):
(IgG) A class of antibodies composed of two identical light and two identical heavy polypeptide chains. IgG acts on antigens by agglutinating (clumping cells together) them. In pregnancy, IgG crosses the placenta to the fetus and protects it against red cell antigens and white cell antigens. Also called gamma globulin.

IMMUNOGLOBULIN M (IgM):
(IgM) A class of antibodies that is made by the body as the initial response to an antigen. If IgM is made in response to a vaccination, a booster shot will result in a "switch" from IgM to mostly immunoglobulin G.

IMMUNOMODULATOR:
Any substance that influences the immune system. See Interleukin-2; Immunostimulant; Immunosuppression.

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

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 (e.g., in chemotherapy) or result from certain disease processes, such as HIV infection.

IMMUNOTHERAPY:
Treatment aimed at reconstituting an impaired 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.

INCIDENCE:
The number of new cases (e.g., of a disease) 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 persons with chronic liver disease or with certain drug allergies; others may exclude men or women, or only include persons with a lowered T cell count.

INCUBATION PERIOD:
The time interval between the initial infection with a pathogen (e.g., HIV) and the appearance of the first symptom or sign of disease.

IND:
See Investigational New Drug.

INDIAN HEALTH SERVICE (IHS):
An agency within the U.S. Department of Health and Human Services responsible for providing federal health services to American Indians and Alaska Natives. The IHS currently provides health services to approximately 1.5 million American Indians and Alaska Natives who belong to more than 557 federally recognized tribes in 34 states. Internet address: http://www.ihs.gov/.

INDINAVIR:
An FDA approved (03/14/96) protease inhibitor for use alone or in combination with nucleoside analogs for the treatment of HIV infection in adults. Also called Crixivan.

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.

INFECTIOUS:
An infection capable of being transmitted by direct or intimate contact (e.g., sex).

INFORMED CONSENT:
The permission granted by a participant in a research study (including medical research) after he/she has received comprehensive information about the study. This is a statement of trust between the institution performing the research procedure and the person (e.g., a patient) on whom the research procedures are to be performed. This includes, for example, the 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 the researchers want to accomplish, (c) what will be done during the trial and for how long, (d) what the risks associated with the trial are, (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. Informed consent also pertains to situations where certain tests need to be performed. See Clinical Trial.

INFUSION:
The process of administering therapeutic fluid, other than blood, 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 or when the drug is too toxic or the volume is too large to be given by quick injection.

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 Vaccine.

INSTITUTIONAL REVIEW BOARD (IRB):
1. A committee of physicians, statisticians, researchers, 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 U.S. must be approved by an IRB before they begin. 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:
A little-understood enzyme that plays a vital role in the HIV-infection process. Integrase inserts HIV's genes into a cell's normal DNA. It operates after reverse transcriptase has created a DNA version of the RNA form of HIV genes present in virus particles. Substances that inhibit integrase are being studied in HIV-infected patients.

INTEGRASE INHIBITORS:
A class of experimental anti-HIV drugs that prevents the HIV integrase enzyme from inserting viral DNA into a host cell's normal DNA.

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 macrophages and brain and nerve cells.

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.

INTERACTION:
See Drug-Drug Interaction.

INTERFERON:
One of a number of antiviral proteins that modulate the immune response. Interferon alpha (IFNa) is secreted by a virally infected cell and strengthens the defenses of nearby uninfected cells. A manufactured version of IFNa (trade names: Roferon, Intron A) is an FDA-approved treatment for Kaposi's Sarcoma, hepatitis B virus, and hepatitis C virus. Interferon gamma is synthesized by immune system cells. (See Natural Killer [NK] Cells and CD4 cells). It activates macrophages and helps orient the immune system to a mode that promotes cellular immunity. (See Thl response).

INTERLEUKINS:
One of a large group of glycoproteins that act as cytokines. The interleukins are secreted by and affect many different cells in the immune system. See Biotechnology; B Lymphocytes; Genetic Engineering; Killer T Cells; Natural Killer Cells (NK); Lymphocyte; T Cells.

INTERLEUKIN-1 (IL-1):
A cytokine that is released early in an immune system response by monocytes and macrophages. It stimulates T cell proliferation and protein synthesis. Another effect of IL-1 is that it causes fever.

INTERLEUKIN-2 (IL-2):
A cytokine secreted by Th1 CD4 cells to stimulate CD8 cytotoxic T lymphocytes. IL-2 also increases the proliferation and maturation of the CD4 cells themselves. During HIV infection, IL-2 production gradually declines. Commercially, IL-2 is produced by recombinant DNA technology and is approved by the FDA for the treatment of metastatic renal (i.e., kidney) cell cancer. Recent data suggest that therapy with subcutaneous IL-2, in combination with antiretroviral drugs, has the potential to halt the usual progression of HIV disease by maintaining an individual's CD4+ T cell count in the normal range for prolonged periods of time.

INTERLEUKIN-4 (IL-4):
A cytokine secreted by Th2 CD4 cells that promotes antibody production by stimulating B cells to proliferate and mature.

INTERLEUKIN-12 (IL-12):
A cytokine released by macrophage in response to infection that promotes the activation of cell-mediated immunity. Specifically, IL-12 triggers the maturation of Thl CD4 cells, specific cytotoxic T lymphocyte responses, and an increase in the activity of NK cells. IL-12 is under study as an immunotherapy in HIV infection.

INTERSTITIAL:
Relating to or situated in the small, narrow spaces between tissues or parts of an organ.

INTRAMUSCULAR (IM):
Injected directly into a muscle.

INTRATHECAL:
Injected into the fluid surrounding the spinal cord.

INTRAVENOUS (IV):
Of or pertaining to the inside of a vein, as of a thrombus. An injection made directly into a vein.

INTRAVENOUS IMMUNOGLOBULIN (IVIG):
A sterile solution of concentrated antibodies extracted from healthy people. IVIG is used to prevent bacterial infections in persons with low or abnormal antibody production. Injected into a vein.

INTRAVITREAL:
Within the eye.

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

INVIRASE:
See Saquinavir.

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 living organisms (e.g., animal or human studies).

IRB:
See Institutional Review Board.

ISOLATE:
An individual (as a spore or a single organism), viable part of an organism (as a cell), or a strain that has been separated (as from diseased tissue, contaminated water, or the air) from the whole. Also, a pure culture produced from such an isolate. A particular strain of HIV taken from a patient.

ITP:
See Immune Thrombocytopenic Purpura.

IVIG:
See Intravenous Immune Globulin.