Radioimmunoassay (RIA) : A very sensitive competitive binding assay that uses radioactivity labeled antigen and specific homologous antibody to determine the amount of the antigen in a sample. The antigen could also be an antibody.

Radioisotope : Unstable molecule that emits detectable radiation (gamma rays, X-rays, etc.) for a known period of time (half-life). Can be incorporated into other compounds as a label for later detection by X-ray film exposure or by measurement in a scintillation counting instrument.

Radiometric analysis : Determination of an element that is not itself radioactive by means of an interaction with a radioactive element.

Racquet Hypha : A series of club-shaped cells in a single hypha.

Radial Immunodiffusion (RID) , is a quantitative variation of the Ouchterlony technique (immunodiffusion) in which the agar gel contains evenly distributed antigen (or antibody) and its counterpart from the test sample diffuses into the gel from a single well resulting in a circular precipitin line around the sample well. The diameter of the precepitin ring is proportional to the concentration of the antibody (or antigen) present in the test sample. By comparing the diameter of the test specimen precipitin ring to known standards, a relatively insensitive estimation of the concentration of specific antibody or antigen can be achieved.

Radioimmunoassay (RIA) , uses fixed-dose, low-level, radioactive-isotope-labeled antigen ("tracer") to compete with unlabeled antigen from the patient specimen for a fixed number of antibody binding sites. Traditional RIA is done with specific antibodies in liquid solution. Solid-phase RIA involves the use of antibody bound to solid support (e.g., tubes, glass beads or plastic fins). The amount of antigen in the specimen is determined by comparing the bound radioactivity with a standard curve.

Radioallergosorbent Test (RAST) , is the name given to an in vitro technique which detects the presence of IgE (and IgG) antibodies to allergens, proteins which may give rise to hypersensitivity reactions seen in allergies. Allergens are coated on a complex carbohydrate matrix called a sorbent. Antibodies specific for the allergen being tested bind to the allergin and, if preset, are detected by a low-level radioactively labeled antibody to either human IgE or IgG, depending upon the isotype being used.

Radioimmunoprecipitation Assay (RIPA) , is the term used to describe the qualitative assay used as a confirmatory procedure for some antibodies to viral antigens. Viral infected cell cultures are radioactively labeled and lysed to yield radiolabeled antigen fragments. Specific antibodies, if present, will bind these antigen fragments and the resulting antigen-antibody complexes are precipitated using protein A, boiled to free the immune complexes which are then separated by electrophoresis. The pattern of antigenic moieties to which antibodies are present may then be detected using autoradiography (the exposure of sensitive X-ray film by the radioactive emissions of the bound, labeled antigens). Comparison to labeled molecular weight standards electroforesed in the same run allows determination of the molecular weight "bands" of antigen to which antibodies are present.

Rales : Abnormal breathing sounds heard by auscultation of the lungs during respiration; classified as dry or moist.

Rash : A superficial skin eruption; may be macular, maculopapular, blotchy, etc.

RAST : Radioallergosorbent test.

RBC : Red blood cell.

REA : Radiative energy attenuation.

Reaction center : A photosynthetic complex containing chlorophyll (or bacteriochlorophyll) and other components, which within occurs the initial electron transfer reactions of photophosphorylation.

Reading-frame shift : See frameshift.

Reagin : An antibody that reacts in various serologic tests for syphilis.

Recalcitrant : Resistant to microbial attack.

Recombinant DNA : A DNA molecule containing DNA originating from two or more sources.

Recombination : Process by which genetic elements in two separate genomes are brought together in one unit.

Rectal prolapse : Extrusion of the rectal mucosa through the anus, may occur in children with heavy Trichuris trichiura infections; usually no sequelae after therapy.

Rectocolitis : Inflammation of both the rectum and colon.

Redia : An elongated sac with a mouth, muscular pharynx, primitive gut, birth pore, and germinal cells, which may develop into cercariae or daughter rediae (stage within snail host: trematodes).

Redox : See oxidation-reduction reaction.

Reduction : A process by which a compound accepts electrons to become reduced.

Reduction potential : The inherent tendency of a compound to act as an electron donor or an electron acceptor. Measure in volts.

Reductive dechlorination : Removal of Cl as Cl- from an organic compound by reducing the carbon atom from C-Cl to C-H.

Regulation : Processes that control the rates of synthesis of proteins. Induction and repression are examples of regulation.

Regulon : A set of operons that are all controlled by the same regulatory protein (repressor or activator).

Renal : Pertaining to the kidney.

Repellent : A chemical applied to the skin or clothing or other places to discourage (1) arthropods from alighting on and attacking an individual, or (2) other agents, such as helminth larvae, from penetrating the skin.

Replacement vector : A cloning vector, such as a bacteriophage, in which some of the DNA of the vector can be replaced with foreign DNA.

Replication : Conversion of one double-stranded DNA molecule into two identical double-stranded DNA molecules.

Report of a disease : An official report notifying an appropriate authority of the occurrence of a specified communicable or other disease in humans or in animals. Diseases in humans are reported to the local health authority; those in animals, to the livestock, sanitary, veterinary or agriculture authority. Some few diseases in animals, also transmissible to humans, are reportable to both authorities.

Repression : The process by which the synthesis of an enzyme is inhibited by the presence of an external substance, the repressor.

Repressor protein : A regulatory protein that binds to specific sites on DNA and blocks transcription.

Reservoir : Source from which an infectious agent may be disseminated; for example, humans are the only reservoir for Mycobacterium tuberculosis.

Resin : Plant product composed largely of esters and ethers of organic acids and acid anhydrides.

Resistance : The sum total of body mechanisms that interpose barriers to the invasion or multiplication of infectious agents, or to damage by their toxic products. Inherent resistance - an ability to resist disease independent of immunity or of specifically developed tissue responses; it commonly resides in anatomic or physiologic characteristics of the host and may be genetic or acquired, permanent or temporary. (See Immunity) (Synonym Nonspecific Immunity).

Respiration : An energy-yielding metabolic process in which electrons from an oxidizable substrate are transferred by a series of oxidatin-reduction reactions (i.e., via an electron-transport chain) to an exogenous terminal electron acceptor such as oxygen, nitrate, or fumarate). (Contrast with fermentation).

Restriction endonuclease : Enzyme that breaks nucleic acid (usually DNA) at only one specific sequence of nucleotides.

Reticulate body : The metabolically more active form of elementary bodies of Chlamydia spp.

Reticuloendothelial system (RES) : Macrophage system, which includes all the phagocytic cells of the body except for the granulocytic leukocytes.

Retinal : Referring to the retina.

Retinitis : Inflammation of the light-detecting tissue (retina) in the back of the eye.

Retinochoroiditis : Inflammation of both the retina and the choroid/vascular coat of the eye (toxoplasmosis, onchocerciasis, visceral larva migrans).

Retrovirus : Virus containing single-stranded RNA as its genetic material and which produces a complementary DNA by action of the enzyme reverse transcriptase.

Reverse electron transport : The energy-dependent movement of electrons against the thermodynamic gradient to form a strong reductant from a weaker electron donor.

Reverse Transciptase PCR (RT-PCR) , is a technique used to amplify RNA targets. The specimen containing the target RNA (e.g., HIV-1 RNA, Hepatitis C Virus RNA) is subjected to reverse transcription to make complementary DNA (cDNA), which is then, in turn, amplified by PCR.

Reverse transcription : The process of copying information found in RNA into DNA.

R factor : Plasmid that carries gene coding for resistance to one or more antibacterial agents.

RFU : Relative fluorescent units.

Rhabditoid (rhabditiform) larvae : Noninfective, thick, rod-shaped larvae of hookworm and Strongyloides stercoralis; rhabditoid refers to the shape of the larval esophagus.

Rh : Symbol of rhesus factor; symbol for rhodium.

RhA : Rheumatoid arthritis.

Rheumatic fever : An inflammatory autoimmune disease triggered by an immune response to infection by Streptococcus pyogenes.

Rheumatoid factor : IgM antibodies produced by some patients against their own IgG.

Rhinitis : Inflammatory or infectious process involving the mucous membranes lining the nose.

Rhinorrhea : Thin, watery discharge from the nose; runny nose.

Rhizoids : The rootlike hyphae occurring at nodes of stolons in some fungi. Example: Rhizopus.

Rhizosphere : The region immediately adjacent to plant roots.

Rhonchi : Coarse, low-pitched sounds associated with the presence of secretions or obstruction of larger airways in the lungs during inspiration and expiration.

RIA : Radiometric immunoassay (radioimmunoassay).

Riboflavin : A heat stable factor of the vitamin B complex , found in milk, leafy vegetables, fresh meat, and egg yolks.

Ribonucleic acid (RNA) : A polymer of nucleotides connected via a phosphate-ribose backbone, involved in protein synthesis.

RID : Radial immunodiffusion.

Rigor : Episodes of rigidity and chills that are occasionally brought on by infections and high fevers.

Ring-enhancing lesions : Lesions in the brain seen on computed tomography (CT) scans, consisting of lucent (less dense to X-rays), rounded masses surrounded by a region of increased density (especially with the use of vascular contrast material); often seen in certain infections of the brain.

Ringworm : Superficial skin disease caused by dermatophytes. Term derived from the ring-like, circular form of the lesions and from the belief that these infections were caused by worm-like organisms. The current accepted term is tinea.

RIP : Radioimmunoprecipitation.

RIPA : Radioimmunoprecipitation Assay.

RNA Polymerase : An enzyme that catalyzes the synthesis of ribonucleic acid (RNA) by using deoxyribonucleic acid (DNA) as a template.

Robisomal RNA (rRNA) : Types of RNA found in the ribosome; some participate actively in the process of protein synthesis.

Ribosome : A cytoplasmic particle composed of ribosomal RNA and protein, which is part of the protein-synthesizing machinery of the cell.

Ribozyme : An RNA molecule that can catalyze a chemical reaction.

Rickettsias : Obligate intracellular parasites which cause a variety of disease, including typhus and Rocky Mountain spotted fever.

RNA life : A hypothetical life form lacking DNA and protein which may have existed on early earth and in which RNA served both a genetic coding and a catalytic function.

RNA processing : The conversion of a precursor RNA into its mature form.

Rod : See bacillus.

Rodenticide : A chemical substance used for the destruction of rodents, generally through ingestion. (See Fumigation).

Romana's sign : Marked edema of one or both eyes; usually dry and unilateral (Chagas' disease).

Root nodule : A tumor-like growth on plant roots that contains symbiotic nitrogen-fixing bacteria.

Rosette : Arrangement of merozoites in mature schizont of Plasmodium malariae in a circle around a clump of excess pigment; also uterine arrangement in the proglottid of Diphyllobothrium latum.

RPR : Rapid plasma reagin, nontreponemal test for antibodies developed in response to syphilis infection.

rRNA : Ribosomal RNA.

RSV : Respiratory syncytial virus, Rous sarcoma virus.

RT-PCR : Reverse transcriptase PCR.

Rumen : The forestomach of ruminant animals in which cellulose digestion occurs.

RVVC : Recurrent vulvovaginal candidiasis.