A

Adherence
Following a prescribed treatment regimen, including correct dosage, number of doses per day, and dietary restrictions.

Adverse Reaction (Adverse Event, Side Effect)
An unwanted toxic reaction to or side effect of drug treatment (e.g., rash, nausea, peripheral neuropathy, bone marrow suppression, liver, or kidney failure).

Aerobic
Requiring or living in the presence of oxygen (e.g., aerobic bacteria).

AIDS Clinical Trials Group (ACTG)
A NIAID-sponsored group of medical centers, known as AIDS Clinical Trials Units (ACTU), that evaluate treatments for HIV disease and associated illnesses. ACTG studies are designated by number (e.g., ACTG 019).

AIDS-Defining Illness
A condition (e.g., Pneumocystis Carinii pneumonia, toxoplasmosis, Kaposi's sarcoma) that is included in the Center for Disease Control and Prevention's (CDC's) definition of AIDS.

Anemia
Reduced ability of blood to carry oxygen due to a low hemoglobin level, or an abnormality or reduced number of red blood cells. Symptoms include fatigue and weakness.

Antibody Positive
Having antibodies (e.g., against HIV) in the blood, indicating previous exposure to an organism. Typically the terms seropositive and HIV positive are used to mean antibody positive.

Antigen (adjective Antigenic)
Any agent or substance that stimulates an immune response.

Antiretroviral
An agent (e.g., AZT, ddI, nevirapine) that suppresses the activity or replication of retroviruses such as HIV.

Area Under the Curve (AUC)
A measure of drug concentration.

Arm
A group of participants in a clinical trial who receive the same treatment (treatment arm) or placebo (control arm).

Assay
A test, especially one used to detect the presence and/or concentration of a component, drug, or microorganism in the blood or other body fluids or tissues.

As-Treated Analysis
A method of analyzing the results of a clinical trial in which only participants who successfully complete a full course of treatment are analyzed.

B

Baseline
An initial or known value (e.g., CD4 cell count, HIV viral load) against which later measurements can be compared.

Bioavailability
The degree to which a drug or other substance is absorbed and circulated in the body; the physiological availability of a drug, as opposed to its chemical potency.

Blinding
A method of conducting clinical trials so that some or all of the participants do not know whether subjects are taking active drug or placebo. Blinding is done to reduce bias in drug trials.

Body Fat Redistribution Syndrome (BFR)
A set of symptoms related to the loss of fat from certain areas of the body such as the limbs and face (lipodystrophy), possibly accompanied by fat accumulation in other areas such as the abdomen (protease paunch) or back of the neck (buffalo hump).

C

CD4 Cell (CD4 Lymphocyte, T-Helper Cell, T4 Cell)
A type of white blood cell that carries the CD4 cell surface receptor and helps the body fight infection. CD4 cells release cytokines (chemical messengers) that coordinate a broad range of immune system activities including killer cell activation and antibody production.

CD8 Cell (CD8 Lymphocyte, T8 Cell)
A type of white blood cell that carries the CD8 cell surface marker and helps regulate and/or carry out the body's immune response. Two subsets of T-cells express the CD8 surface marker T-suppressor cells and cytotoxic T-lymphocytes (CTLs).

Cell Line
A specific type of cell that is maintained in vitro for medical and/or research purposes.

Cervicovaginal
Relating to the uterine cervix and the vagina, parts of the female reproductive system.

Cervix (adjective Cervical)
The cylindrical, lower part of the uterus leading into the vagina.

Chemokine
A soluble factor secreted by certain immune system cells that stimulates the activity of other cells. Chemokines have chemoattractant properties and act as messengers between cells. Certain chemokines (e.g., MIP-1 alpha, MIP-1 beta, RANTES) have been shown to affect the activity of HIV; certain chemokine receptors (e.g., CCR5, CXCR4) are necessary to allow HIV to enter host cells.

Cholesterol
A fatty substance in animal tissue that is an essential component of cell membranes and nerve fiber insulation. Cholesterol is important for the metabolism and transport of fatty acids and in the production of hormones and vitamin D. Cholesterol is manufactured by the liver, and is also present in certain foods (e.g., eggs, shellfish). There are two primary types of cholesterol in the blood: low-density lipoprotein (LDL), which is considered a risk factor for heart disease, and high-density lipoprotein (HDL), which is considered protective against heart disease. Very low cholesterol levels may indicate malnutrition.

Chronic
Persisting or recurring over a long period of time; marked by slowly progressing seriousness.

Coenzyme Q10 (CoQ10)
A quinone substance that facilitates the production of energy from food and acts as an antioxidant.

Cohort
A group of individuals in a study who share a demographic, clinical, or other statistical characteristic (e.g., age, study site).

Combination Therapy
Simultaneous or alternating administration of two or more therapies.

Coreceptor
A second cell surface receptor required for entry by a pathogen into a host cell or for initiation of a biological process. HIV requires both the CD4 receptor and a coreceptor (e.g., CCR-5 or CXCR-4) to enter a cell.

Cytochrome P450 System
A system that facilitates oxidative metabolism drugs and other substances in the liver by means of enzymes (e.g., CYP3A4) that inhibit or promote metabolic reactions.

Cytokine
An intercellular hormone or chemical messenger protein (e.g., tumor necrosis factor, interleukin) released by white blood cells (e.g., macrophages, T-cells). Cytokines facilitate communication among immune system cells and between immune system cells and the rest of the body. Cytokines coordinate several aspects of the immune response including stimulating antibody production, promoting the migration of phagocytes to the site of an infection, and activating cytotoxic T-lymphocytes.

Cytotoxic T-Lymphocyte (CTL, Killer T-Cell)
A type of white blood cell that bears the CD8 surface marker, and targets and kills cells infected with viruses, bacteria, parasites, and other microorganisms.

D

Dendritic Cell
A type of immune system cell with many branches. Dendritic cells are found in the skin and mucosal membranes. They are typically the first to arrive at sites of injury or infection, where they bind to invaders and transport them to the lymph nodes where the cell-mediated immune response is initiated. Dendritic cells carry the CD4 surface marker, and may be among the first cells to be infected by HIV.

Deoxyribonucleic Acid (DNA)
A molecule that encodes genetic information and is found in the nucleus of cells as a twisted double-stranded chain. The particular sequence of four chemical building blocks (nucleotides) -- adenine, cytosine, guanine, and thymine -- that make up a DNA chain determines the unique genetic code of an individual. See also ribonucleic acid.

Dexa Scan
Dual energy X-ray absorptiometry, a method of determining the composition of body tissues.

Discordant
Not having the same serostatus, for example a couple in which one partner is HIV positive and the other is HIV negative.

DNA
See deoxyribonucleic acid.

Double-Blind
A type of clinical trial in which neither the subject nor the investigator knows what treatment, if any, the subject is receiving.

E

ELISA
See enzyme-linked immunosorbent assay.

Endocervix (adjective Endocervical)
The inner part of the uterine cervix.

ENV
The gene of HIV that encodes the proteins of the viral envelope; also refers to the proteins produced by that gene.

Enzyme
A protein that induces or accelerates a chemical reaction.

Enzyme-linked Immunosorbent Assay (ELISA)
A laboratory test used to detect the presence of antibodies in the serum. ELISA is used for first-line screening for HIV antibodies; a positive result indicates that antibodies have been detected. The test is sensitive but not specific, and a positive ELISA is typically confirmed using a Western blot assay.

Etiology
The cause of a disease; the study of causes of disease.

F

First-line Treatment
The preferred standard therapy for a particular condition.

G

GAG
The gene of HIV that encodes the core proteins of the virus; also refers to the proteins produced by that gene.

Gene (adjective Genetic)
The unit of heredity. A gene contains hereditary information encoded in the form of DNA and is located at a specific position on a chromosome in a cell's nucleus. Genes determine many aspects of anatomy and physiology by controlling the production of proteins.

Genome (adjective Genomic)
The unique genetic code or hereditary material of an organism, carried by a set of chromosomes in the nucleus of each cell. The human genome contains an estimated 50,000-100,000 genes; the genome of HIV contains nine genes.

Genotype (adjective Genotypic)
The specific genetic makeup or "blueprint" of an organism. Genotypic resistance testing determines whether an organism's genetic structure contains certain mutations that make it resistant to a drug. Contrast with phenotype.

H

HAART
Highly active antiretroviral therapy, a term for potent combination anti-HIV treatment that usually includes a protease inhibitor.

Half-life
The time required for half of the amount of an agent (e.g., drug, virus, cell type) to be eliminated from the body.

Hepatic
Relating to the liver.

Hepatitis
An inflammation of the liver that may be caused by several agents, including viruses, toxins, or drugs. Acute hepatitis is typically characterized by fever, jaundice, fatigue, abdominal tenderness, and elevated liver enzymes. Hepatitis A virus (HAV) causes an inflammatory viral disease with a short incubation period. Hepatitis viruses B (HBV) and C (HCV) can become chronic and may result in liver damage, cirrhosis, or liver cancer. There are also hepatitis viruses D, E, F, and G.

Hepatomegaly
Liver enlargement.

Herpes Simplex Virus (HSV-1, HSV-2, Human Herpesvirus 1, 2)
A herpesvirus that causes recurring lesions. Herpes simplex virus type 1 (HSV-1) usually produces blisters on the lips or in the mouth (cold sores or fever blisters). HSV type 2 (HSV-2) is usually sexually transmitted and generally produces lesions in the genital or anal area. Both HSV-1 and HSV-2 are treated with acyclovir; ganciclovir or foscarnet are used to treat acyclovir-resistant HSV.

Hormone
A chemical messenger (e.g., adrenaline, testosterone) involved in the regulation and coordination of cellular and bodily functions.

I

Immunology (adjective Immunologic)
The study and/or treatment of disorders that involve the immune system (e.g., cancer, HIV disease, autoimmune diseases).

Incidence (Incidence Rate)
The number of new cases of a disease or condition in a specific population during a given period of time. The incidence rate is determined by dividing the number of new cases by the total population.

Incubation Period
The period of time between an initial exposure to an infectious agent and the development of symptoms of disease, during which the infection is established in the body.

Insulin
A peptide hormone produced in the pancreas. Insulin enables the body to metabolize and use glucose (sugar). Lack of or insensitivity to insulin results in diabetes.

Intent-to-Treat Analysis
A method of analyzing the results of a clinical trial in which all participants originally assigned to an arm are analyzed, including those who dropped out due to treatment failure or side effects.

Interferon-Alpha (Intron-A, Roferon-A)
A type of interferon produced in response to viral infection. Genetically engineered interferon-alpha is used to treat Kaposi's sarcoma, anogenital warts, and chronic hepatitis B and C, and is under study as an anti-HIV therapy. Side effects include flu-like symptoms, anemia, and neutropenia.

In Vitro
Latin for "in glass." Refers to studies done in a test tube or culture medium in the laboratory.

In Vivo
Latin for "in the body of a living organism." Refers to studies done using human or animal subjects.

K

Kaposi's Sarcoma (KS)
An abnormal or cancerous proliferation of cells of the blood and/or lymph vessels causing tumors on the skin, mucous membranes, and/or internal organs. KS typically appears as purplish or brownish lesions.

L

Lactic Acidosis
A buildup of lactic acid (a byproduct of carbohydrate metabolism) in bodily tissues.

Latency (adjective Latent)
The state in which a pathogenic organism is present in the body but not actively replicating or causing illness.

Lean Body Mass
Muscle and organ tissue.

Leukopenia
An abnormally low number of white blood cells in the circulating blood.

Lipodystrophy
A loss of fatty tissue, usually in the limbs and face. The term is also sometimes used to refer to any type of body fat redistribution.

Log
Refers to quantities in factors of ten. A log change is an exponential or ten-fold increase or decrease (e.g., a change from ten to 100 is a 1-log increase).

Long-Term Nonprogressor (LTNP)
An individual who has been infected with HIV for several years (7-10 or more years) but does not exhibit immune system decline or have opportunistic diseases. About 10% of persons with HIV disease seem to be LTNPs.

Lymphocyte
A type of white blood cell (e.g., T-cell, B-cell, natural killer cell) that plays a part in immune defense.

M

Macaque
A family of monkey species (e.g., rhesus, pigtail, cynomolgus) often used in scientific research. Macaques can be infected with simian immunodeficiency virus (SIV) and are used as an animal model for AIDS.

Macrophage
A large scavenger white blood cell that ingests and processes degenerated cells and foreign invaders. The long-lived macrophages can harbor large quantities of HIV, and may act as viral reservoirs.

Magnetic Resonance Imaging (MRI)
A sensitive, noninvasive method for viewing soft tissues of the body using a strong magnetic field.

MBChB
Bachelor of Medicine, Bachelor of Surgery.

Microbe
A tiny living organism (e.g., bacterium, protozoan, fungus), especially those that cause disease.

Microbicide
An agent that inactivates, kills, or destroys microbes. Vaginal and anal microbicides are under study as a means of preventing HIV transmission.

Micronutrient
A trace element; an organic compound, small amounts of which are essential for physical health, growth, and metabolism.

Mitochondria (singular Mitochondrion)
Rod-shaped or oval organelles in the cytoplasm of a cell that produce most of the cell's energy.

Monocyte
A large white blood cell that plays a role in immune defense. Monocytes circulate in the bloodstream; when they migrate to the tissues, they mature into macrophages.

Multivariate Analysis
A statistical analysis technique in which multiple variables are analyzed separately to determine the contribution made by each variable to an observed result. Contrast with univariate analysis.

Mutation
A change in the character of a gene that is perpetuated when a cell divides or a virus replicates.

N

Nadir
The lowest point. The term is used to refer to the lowest CD4 cell count a person ever reaches before the count increases due to anti-HIV treatment.

Naive
Inexperienced. The term is used to describe an individual who has never taken a certain drug or class of drugs (e.g., AZT-naive, antiretroviral-naive) or to refer to an undifferentiated immune system cell.

Natal
Relating to birth.

Natural Killer Cell (NK Cell)
A type of white blood cell that attacks and kills tumor cells and cells infected with microorganisms. Unlike cytotoxic T-lymphocytes, NK cells are nonspecific and attack infected cells without regard to specific antigens or MHC receptor configuration.

NEF
A gene of HIV that influences viral replication and may help HIV evade host defenses; also refers to the protein produced by that gene.

Neuropathy (adjective Neuropathic)
Damage to the nerves; an inflammatory or degenerative condition of the nervous system. See also peripheral neuropathy.

Non-Nucleoside Reverse Transcriptase Inhibitor (NNRTI)
A drug (e.g., delavirdine, efavirenz) that binds with and inhibits the action of the HIV reverse transcriptase enzyme, thus blocking viral transcription and replication.

Nucleoside Analog
(also known as Nucleoside Reverse Transcriptase Inhibitor, NRTI)
A compound (e.g., AZT, d4T, 3TC) that mimics one of the building blocks of DNA. These compounds suppress retroviral replication by interfering with the reverse transcriptase enzyme.

O

Open-Label
Refers to a drug trial that is not randomized and not blinded; both participants and investigators know what drug is being tested and what dosages are being used.

Opportunistic Infection (OI)
A disease caused by a microorganism that does not normally cause illness in a person with a healthy immune system, but that may cause serious disease when the immune system is weakened. Common OIs in HIV positive people include Pneumocystis carinii pneumonia (PCP), Mycobacterium avium complex (MAC), and cytomegalovirus (CMV) infection.

Organelle
One of the specialized small organs of a living cell (e.g., mitochondria).

Oxidative Stress
Increased levels of free radicals and other oxidation-promoting molecules associated with disease, immune response, and aging. When the production of pro-oxidants exceeds the cellular supply of antioxidants, harmful effects may result including cell membrane damage, cell death, and damage to genetic material resulting in mutations.

P

P450
See cytochrome P450 system.

Pancytopenia
An abnormally low level of all types of blood cells, typically due to bone marrow damage.

Peripheral Neuropathy
Damage to the peripheral nerves, usually involving the feet and hands, and sometimes the legs, arms, and face. Symptoms may include numbness, tingling or burning sensations, pain, abnormal reflexes, weakness, and partial paralysis. Peripheral neuropathy is a side effect of HIV as well as several anti-HIV drugs (e.g., ddC, ddI, and d4T).

pH
A logarithmic scale used to describe the acidity or alkalinity of a solution; a measure of the hydrogen concentration of a solution. Water has a neutral pH of 7. A pH below 7 is acidic; a pH above 7 is alkaline (or basic).

Phenotype (adjective Phenotypic)
Visible characteristics and/or behavior that result from the interaction of an organism's genetic "blueprint" (genotype) and the environment. Phenotypic resistance testing determines whether an organism is susceptible to a specific drug in a test tube. Contrast with genotype.

Phosphorylation
The addition of a phosphate group to a compound by an enzyme (e.g., thymidine kinase, tyrosine kinase). Phosphorylation is an essential step in many cellular processes.

Placebo
An inert, inactive substance; a "sugar pill." Experimental therapies are compared with placebo in many clinical trials.

Plasma
The fluid, non-cellular portion of circulating blood that carries blood cells and nutrients throughout the body, removes metabolic wastes, and is a medium for chemical communications between different parts of the body.

Postexposure Prophylaxis (PEP)
Drug therapy given immediately following exposure to an infectious organism, done in an attempt to prevent the infection from taking hold in the body.

Prenatal
The period preceding birth, during which the fetus develops in the uterus.

Prevalence (Prevalence Rate)
The proportion of individuals with a condition in a specific population during a given time. The prevalence rate is determined by dividing the number of people with the condition by the total population.

Prodrug
An inactive form of a drug that exerts its effects after metabolic processes within the body convert it to a usable or active form.

Progesterone
A female steroid hormone with anti-estrogen effects. Progesterone prepares the uterus for the development of the fertilized ovum and maintains the uterus throughout pregnancy. Synthetic progesterone is used as a contraceptive.

Progestin
A hormone produced by the corpus luteum, placenta, or adrenal cortex (or synthetically manufactured) that has progesterone-like effects. Synthetic progestin is used as a contraceptive.

Proliferation
Rapid reproduction or replication.

Prospective Study
A study that looks forward in time. Participants are selected and their progression is followed over time.

Protease Inhibitor (PI)
A drug (e.g., saquinavir, ritonavir, indinavir, nelfinavir, amprenavir) that blocks the action of the HIV protease enzyme, thereby preventing viral replication. Unlike reverse transcriptase inhibitors, protease inhibitors can inhibit HIV replication in cells that are already infected.

Protocol
A plan that states the specifics of a clinical trial, such as the hypothesis to be tested, drug(s) to be used, method(s) of administration, length of the trial, endpoint(s) to be used, and characteristics of participants (e.g., inclusion/exclusion criteria).

Q

Quiescent
Refers to a state of resting, inactivity, or latency.

R

Randomized Trial
An experiment in which subjects with particular characteristics are assigned by chance to different treatment or control arms. Randomization is done to cancel out the influence of factors that are not under study.

Rebound
An increase in viral load following a previous decrease due to anti-HIV therapy.

Receptor
A specific binding site on a cell's surface or interior. When chemical messengers bind to receptors, various cellular functions are activated or inhibited. Many drugs exert their effects by binding to receptors and altering normal cellular communication. Viruses enter cells by fusing with receptors on the cell surface, often in conjunction with certain genes (MHC molecules) and/or coreceptors.

Renal
Pertaining to the kidneys.

Reservoir
A site where an infectious agent collects and multiplies; resting CD4 cells are reservoirs of HIV.

Resistance
The mutation of a microorganism in such a way that it loses its sensitivity to a drug.

Reverse Transcriptase (RT)
A viral enzyme that allows a retrovirus to translate its genetic material (in the form of RNA) into DNA, which is then integrated into the chromosomes of the host cell.

Ribonucleic Acid (RNA)
A single-stranded nucleic acid that encodes genetic information. RNA is made up of a sequence of four chemical building blocks (nucleotides): adenine, cytosine, guanine, and uracil. RNA is involved in the transcription of genetic information; the information encoded in DNA is translated into messenger RNA (mRNA), which controls the synthesis of new proteins. RNA takes the place of DNA in retroviruses such as HIV. The presence of HIV RNA in the plasma indicates that the virus is actively replicating. See also deoxyribonucleic acid.

RNA
See ribonucleic acid.

S

Second-Line Treatment
The second preferred therapy for a particular condition used when a first-line treatment fails or when a person cannot tolerate the side effects of first-line drugs.

Semen
A whitish, viscid fluid ejaculated from the penis containing spermatozoa and secretions from the prostate gland.

Sensitivity
The ability of an organism to be affected by a drug or other agent (e.g., a virus is sensitive to AZT if AZT is able to prevent viral replication). Sensitivity also refers to a statistical measure of the accuracy of a screening test, i.e., how likely a test is to label as positive those who have a disease or condition.

Seroconversion
The development of antibodies directed against an antigen; the change in a person's antibody status from negative to positive. Vaccine-induced seroconversion is not an infection.

Seronegative
Having a blood test that does not indicate infection with a microorganism (e.g., HIV), either by the presence of antibodies or antigen.

Seropositive
Having a blood test result that indicates infection with a microorganism (e.g., HIV). A test may detect either antibodies to an organism (antibody positive) or the organism or its proteins (antigen positive).

Sexually Transmitted Disease (STD, Venereal Disease)
A disease (e.g., gonorrhea, syphilis, chlamydiasis, herpes) that is transmitted through sexual contact.

Simian Immunodeficiency Virus (SIV)
A viral infection endemic to African green monkeys and able to infect various primates. SIV is similar to HIV-2.

Somatic
Relating to the physical body.

Spermicide
An agent (e.g., nonoxynol-9) that immobilizes or kills spermatozoa; spermicides are also often used to kill HIV and other organisms.

Standard Therapy
A therapy that is FDA-approved for a specific condition and is widely used as first-line treatment for that condition.

Steatosis
Tissue degeneration (e.g., in the liver) caused by the buildup of fat in the cells.

Strain
A specific genetic variant of a particular organism.

Subcutaneous
Beneath the skin; subdermal.

Subtype (Clade)
A phylogenetically distinct strain of a microorganism. There are at least 11 subtypes of HIV-1 classified into two groups: group M (containing subtypes A through J) and group O. Subtype B is predominant in the U.S. There are five known subtypes of HIV-2 (A-E). See also strain.

Suppression
The cessation of detectable viral replication in a person's blood.

T

T-Cell (T-Lymphocyte)
A type of white blood cell that matures in the thymus and carries out the cell-mediated immune response. Three major types of T-cells are helper (CD4) T-cells, suppressor (CD8) T-cells, and killer (CD8) T-cells (cytotoxic T-lymphocytes, or CTLs).

Third-Line Treatment
The third preferred therapy for a particular condition; used when the patient fails or cannot tolerate the side effects of the first- and second-line treatments.

Thrombocytopenia
An abnormally low number of platelets; the condition may result in abnormal bleeding and bruising. The normal platelet range is 150,000-300,000 per millimeter of blood. Thrombocytopenia may be controlled by the administration of certain cytokines or by removal of the spleen.

Thymus (adjective Thymic)
A lymphoid organ located in the chest behind the sternum. The thymus is a source of T-cell production in the fetus and the site of T-cell maturation and differentiation, i.e., where the cells learn to recognize antigens.

Trichomoniasis (TV)
Infection with the protozoan parasite Trichomonas vaginalis. Trichomoniasis may be asymptomatic, especially in men; in women symptoms may include vaginal irritation, itching, discharge, and painful urination.

Triglyceride
A combination of glycerol and fatty acid that circulates in the blood.

U

Undetectable (Unquantifiable)
A term used to describe a viral load that is below the level of detection of the test being used.

Univariate Analysis
A statistical analysis that considers only one factor or variable at a time. Contrast with multivariate analysis.

Uterus (Womb)
The muscular pelvic organ of the female reproductive system in which the fetus develops.

V

Vagina
The passage from the uterus to the outside of the body, part of the female reproductive system; the "birth canal."

Viral Load (Viral burden)
The amount of virus in the blood or body tissues.

Viremia
The presence of virus in the blood.

Virological Failure
Treatment breakthrough; an increase in viral load after it has been suppressed with antiretroviral drugs.

Virology
The study of viruses and diseases caused by viruses.

Virus
Any of a large group of minute organisms that cannot grow or reproduce outside of a host cell. During replication, a virus integrates its genetic material (DNA or RNA) into the host cell and takes over the cell's biological mechanisms to reproduce new viral particles. Various families of viruses infect humans, animals, plants, and bacteria.

W

Western Blot
A laboratory blood test for specific antibodies (e.g., to HIV). The Western blot assay is more accurate than the ELISA, and is used as a confirmatory test if an ELISA is positive.

Wild Type (adjective Wild-Type)
The normal, typical phenotype of a virus or other organism before genetic mutation takes place.

World Health Organization (WHO)
An organization of the United Nations concerned with worldwide public health.

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