p24: 1. Within the envelope of the HIV virus is a bullet-shaped core made of another protein, p24, that surrounds the viral RNA. 2. The p24 antigen test looks for the presence of this protein in a patient's blood. 3. A positive result for the p24 antigen suggests active HIV replication. p24 found in the peripheral blood is thought to also correlate with the amount of virus in the peripheral blood. It is believed that there are measurable levels of p24 when first infected with the virus after which there is a strong antibody response to p24 in early disease. Low or unmeasurable levels of p24 may indicate that the virus is in a dormant stage. Spikes in p24 levels may indicate that HIV has begun active replication.

PALLIATIVE: A treatment that provides symptomatic relief, but not a cure.

PANCREAS: A gland situated near the stomach that secretes a digestive fluid into the intestine through one or more ducts and also secretes the hormone insulin.

PANCREATITIS: Inflammation of the pancreas that can produce severe pain and debilitating illness. See also Pancreas.

PANDEMIC: A disease prevalent throughout an entire country, continent or the whole world. See also Epidemic.

PAP SMEAR: A method for the early detection of cancer and other abnormalities of the female genital tract, especially of the cervix and uterus, employing exfoliated cells (cells that have been shed into vaginal fluid) and a special staining technique for microscopic examination that differentiates diseased tissue. Also known as Papanicolaou Smear after George Papanicolaou, the American cytologist who developed this method and published it in 1943. See also Cervix; Uterus.

PAPILLOMA: 1. A benign tumor (as a wart or condyloma) resulting from an overgrowth of epithelial tissue on papillae of vascularized connective tissue (as of the skin). 2. An epithelial tumor caused by a virus. See also Condyloma; Epithelium.

PARALLEL TRACK: A system of distributing experimental drugs to patients who are unable to participate in ongoing clinical efficacy trials and have no other treatment options. See also Clinical Trial.

PARASITE: A plant or animal that lives and feeds on or within another living organism; does not necessarily cause disease.

PATHOGEN: Any disease-producing microorganism or material.

PATHOGENESIS: The origin and development of a disease.

PBMC: See Peripheral Blood Mononuclear Cell.

PCP: See Pneumocystis carinii Pneumonia.

PCR: See Polymerase Chain Reaction.

PEER REVIEW: The process by which new scientific or medical findings, announced by one researcher, are reviewed by other scientists or physicians before these findings are published.

PENTAMIDINE: An approved antiprotozoal drug used for the treatment and prevention of Pneumocystis carinii pneumonia (PCP) infection. It can be delivered intravenously or intramuscularly or inhaled as an aerosol. Aerosolized pentamidine is approved for the prophylaxis of PCP in HIV-positive individuals with CD4+ counts below 200 per cubic millimeter or for those with prior episodes of PCP. The drug is also known under the names Pentam and NebuPent. See also Pneumocystis carinii Pneumonia.

PEPTIDE: (Also polypeptide). Biochemical formed by the linkage of up to about 50 amino acids to form a chain. Longer chains are called proteins. The amino acids are coupled by a peptide bond, a special linkage in which the nitrogen atom of one amino acid binds to the carboxyl carbon atom of another. Many peptides, such as the hormones vasopressin and ACTH, have physiological or antibacterial activity. See also Amino Acids.

PERIANAL: Around the anus.

PERIPHERAL BLOOD MONONUCLEAR CELL (PBMC): Cells in the bloodstream with one nucleus. See also Nucleus.

PHAGOCYTE: A cell that is able to ingest and destroy foreign matter, including bacteria.

PHAGOCYTOSIS: The process of ingesting and destroying a virus or other foreign matter by phagocyte. See also Macrophage; Monocyte.

PHARMACOKINETICS: The processes (in a living organism) of absorption, distribution, metabolism and excretion of a drug or vaccine.

PLACEBO: An inactive substance against which investigational treatments are compared for efficacy. See also Placebo Controlled Study.

PLACEBO CONTROLLED STUDY: A method of investigation of drugs in which an inactive substance (the placebo) is given to one group of patients, while the drug being tested is given to another group. The results obtained in the two groups are then compared.

PLACEBO EFFECT: A physical or emotional change occurring after a substance is taken or administered that is not the result of any special property of the substance. The change may be beneficial, reflecting the expectations of the patient and, often, the expectations of the person giving the substance.

PLASMA: That 10 percent of the blood that contains nutrients, electrolytes (dissolved salts), gases, albumin, clotting factors, wastes and hormones.

PLASMA CELLS: Large antibody-producing cells that develop from B cells. See also Antibodies; B Lymphocytes.

PLATELETS: Active agents of inflammation when damage occurs to a blood vessel. They are not actually cells, but fragments released by megakaryocyte cells. Megakaryocyte is a large cell in the bone marrow whose function is to produce platelets. When vascular damage (i.e., damage to blood vessels) occurs, the platelets stick to the vascular walls, forming clots to prevent the loss of blood. Thus, it is important to have adequate numbers of normally functioning platelets to maintain effective coagulation of the blood. There are drugs that can potentially alter the platelet count, making it necessary to monitor the count. Also, some people with HIV infection develop thrombocytopenia (a condition characterized by a platelet count of less than 100,000 platelets per cubic millimeter of blood). The normal value for men is 154,000-354,000 platelets per cubic millimeter of blood. For women, it is 162,000-380,000 platelets per cubic millimeter of blood.

PNEUMOCYSTIS CARINII PNEUMONIA (PCP): 1. A protozoal infection of the lungs. 2. A life-threatening lung infection that can affect people with weakened immune systems, such as those infected with HIV. More than three-quarters of all people with HIV disease will develop PCP if they do not receive treatment to prevent it. The standard treatment for people with PCP is either a combination of trimethoprim and sulfamethoxazole (TMP/SMX, also called Bactrim or Septra) or pentamidine. See also Pentamidine; Protozoa.

POLYMERASE: Any of several enzymes that catalyze the formation of DNA or RNA from precursor substances in the presence of preexisting DNA or RNA acting as templates (i.e., patterns). See also DNA; Enzyme; Ribonucleic Acid.

POLYMERASE CHAIN REACTION (PCR): 1. A laboratory process that selects a DNA segment from a mixture of DNA chains and rapidly replicates it; used to create a large, readily analyzed sample of a piece of DNA. It is used in DNA fingerprinting and in medical tests to identify diseases from the infectious agent's DNA. See also DNA. 2. As related to HIV: A sensitive laboratory technique that can detect and quantify HIV in a person's blood or lymph node.

PREVALENCE: A measure of the proportion of people in a population affected with a particular disease at a given time.

PROPHYLAXIS: Treatment that helps to prevent a disease or condition before it occurs or recurs.

PROTEASE: An enzyme that hydrolyzes (i.e., breaks down) proteins to their component peptides. See also Enzyme; Peptide; Proteins.

PROTEASE INHIBITORS: HIV protease is an aspartyl enzyme essential to the replicative life cycle of HIV. The three-dimensional molecular structure of the HIV protease has been fully determined. Pharmaceutical developers are therefore able to rationally design compounds to inhibit it and thus interfere with replication of the virus. In the US, five peptide-based protease inhibitors (saquinavir, Roche; A-80987, ABT-538, Abbott Laboratories; L735,524, Merck; KNI-272, NCI) are in clinical development. All compounds inhibit HIV-1 in vitro in nanomolar concentrations. In Europe, two peptide-based compounds (ABT-987, Abbott Laboratories; AG-1343, Agouron Pharmaceuticals, Inc.) are currently in development. See also In Vitro.

PROTEINS: Any of the group of highly complex organic compounds found in all living cells. Protein is the most abundant class of all biological molecules, comprising about 50 percent of cellular dry weight. Classified by biological function, proteins include the enzymes, which catalyze cellular reactions; collagen, keratin and elastin, which are structural, or support, proteins; hemoglobin and other transport proteins; casein, ovalbumin and other nutrient proteins; antibodies, which are necessary for immunity; protein hormones, which regulate metabolism; and proteins such as actin and myosin, the contractile muscle proteins that perform mechanical work. Structurally, proteins are large molecules composed of one or more chains of varying amounts of the same 22 amino acids, which are linked by peptide bonds. Each protein is characterized by a unique and invariant amino acid sequence. Protein chains may contain hundreds of amino acids; some proteins also incorporate phosphorus or such metals as iron, zinc and copper. The amino acid sequence also determines the molecule's three-dimensional structure; this so-called native state is required for proper biological function. The information for the syntheses of the specific amino acid sequences from free amino acids is carried by the cell's nucleic acid. See also Peptide; Ribonucleic Acid.

PROTOCOL: The detailed plan for a clinical trial that states the trial's rationale, purpose, drug or vaccine dosages, length of study, routes of administration, who may participate and other aspects of trial design. See also Clinical Trial; Inclusion/Exclusion Criteria.

PROTOPLASM: Fundamental material composing all living things. Protoplasm, which exists in all plants and animals in the small units called cells, is mainly (85-90 percent) water and also contains proteins, fatty substances and inorganic salts. It is always enclosed by a thin surface membrane that controls the passage of materials into and out of the cell. It displays the general properties associated with life-the capacity to respond to stimuli and the ability to perform the essential physiological functions.