Icosahedron : A geometrical shape occurring in many virus particles, with 20 triangular faces and 12 corners.

ICT : Inflammation of connective tissue.

Icterus : See jaundice.

i.d. : Intradermal.

ID : Infectious disease; identification; immunodiffusion.

Idiopathic cardiomypathy : Dilatation and weakening of the heart muscle with no known cause.

IE : Infectious endocarditis.

IEF : Isoelectricfocusing.

IEP : Immunoelectrophoresis.

IF : Immunofluorescence.

IFA : Indirect fluorescent antibody; test that detects antibody by allowing an antibody to react with its substrate and adding a second fluorescein dye-labelled antibody that will bind to the first.

IFE : Immunofixation electrophoresis.

Ifix : Immunofixation.

IFN : Interferon.

Ig, IgG, etc : Immunoglobulin, immunoglobulin G, etc.

IgM Antibody Capture ELISA (MAC ELISA) , has been developed to impart significant improvement in assay specificity to indirect ELISA procedures for IgM isotype antibodies. Solid-phase support (usually microtiter plate wells) are coated with anti-human IgM antibodies capable of binding all IgM isotype antibodies present in the specimen. Reagent antigen is then added, followed by enzyme-labeled antigen-specific antibodies. If IgM antibodies specific for the antigen in question are present, the "sandwich" complex will result in enzymatic color-change proportional to the concentration of IgM-specific antibody present. This technique appears to be the method of choice in many highly specific and more assays for IgM infectious disease antibodies.

IH : Infectious hepatitis.

IHA : Indirect hemagglutination.

IL-2 : Interleukin-2.

IM; i.m. : Intramuscular, infectious mononucleosis.

Immobilized enzyme : An enzyme attached to a solid support over which substrate is passed and is converted into product.

Immune1 (adjective) : When a human or animal body is able to resist infectious disease because of either previous infection, immunization/vaccination, or contact with the immunogen/antigen, then the human or animal is immune to this disease.

Immune2 (noun) : The state of immunity.

Immune individual : A person or animal that has specific protective antibodies and/or cellular immunity as a result of previous infection or immunization, or is so conditioned by such previous specific experience as to respond in such a way that prevents the development of infection and/or clinical illness following re-exposure to the specific infectious agent.

Immune response : The specific reactions induced in the human or animal body due to the contact with foreign material. The foreign material which induces the immune response is called an immunogen or antigen. The immunse response may involve either antibody production, the activation of T cells or both.

Immunity : The ability of an individual to resist and/or control the effects of antigens (antigen sources could be animal, plant, or mineral). Effective immunity includes both cellular immunity, which is conferred by T-lymphocyte sensitization, and/or humoral immunity, which is based on B-lymphocyte response. Passive immunity is attained either naturally by transplacental transfer from the mother, or artificially by inoculation of specific protective antibodies (from immunized animals, or convalescent hyperimmune serum or immune serum globulin [human]); it is of short duration (days to months). Active humoral immunity, which usually lasts for years, is attained either naturally by infection with or without clinical manifestations, or artificially by inoculation of the agent itself in killed, modified or variant form, or of fractions or products of the agent.

Immunization : Induction of specific immunity by injecting antigens, antibodies, or immune cells.

Immunoblot , commonly referred to as "Western Blot" (WB) because of the similarity to Southern Blot (SB) and Northern Blot (NB), is used to detect antibodies to specific epitopes of electrophoretically separated subspecies of antigens. Electrophoresis of antigenic material yields separation of the antigenic components by molecular weight. Blotting of the separated antigen to nitrocellulose, retaining the electrophoretic position, and reacting it with patient specimen will result in binding of specific antibodies, if present, to each antigenic "band". Electrophoresis of known molecular weight standards allows for the determination of the molecular weight of each antigenic band to which antibodies may be produced. These antibodies are then detected using EIA reactions which characterize antibody specificity. This technique is often used to confirm the specificity of antibodies which are detected by ELISA screening procedures.

Immunocytochemical Assay (ICA) , involves the computerized assessment of microscopic fields following Direct Flourescent Antibody (DFA), Indirect Fluorescent Antibody (IFA) or Indirect Immunoperoxidase (IP) analysis of biopsy tissue from the patient. In addition to improved specificity with the removal of operator subjectivity, the quantifiability of results through computer data analysis of color, intensity and concentration has only begun to be realized.

Immunodiffusion (ID) , also called Double Diffusion (DD) or the Ouchterlony technique, is the classical procedure used to detect the presence of antibodies and determine their specificity by visualization of "lines of identity" (precipitin lines). These precipitin lines (precipitated antigen-antibody complexes) form where the binding concentrations of antigen and antibody are equivalent. Patient serum diffuses from one well through the gel and reacts with a known specific antigen (or antibody) which diffuses through the gel from a second well. DD is strictly qualitative, although the density of the precipitin line and the distance of the line from the sample well may give some identification of the antibody concentration.

Immunoelectrophoresis (IEP) , is a two-step procedure which first involves the electrophoretic separation of proteins, followed by the linear diffusion of antibodies into the
electrophoretic gel from a trough which extends through the length of the gel adjacent to the electrophoretic path. The antigen-antibody reactions produce precipitin arcs at positions where equivalence occurs. Although quantitation is subjective, an experienced eye can determine not only the presence of the antigen but, through visual comparison to normal control sera, may discriminate relative increases or decreases of antigen by gauging the length and density of the precipitin arcs at positions established for specific antigens using known standards.

Immunofixation (IFIX) , is a powerful enhancement of immunoelectrophoresis in which a series of post-electrophoretic gel slabs are layered with cellulose-acetate gels saturated with specific antibodies. The resulting antigen-antibody complexes fixed on the second gel may then be stained, allowing sensitive and specific qualitative visual identification of paraproteins by electrophoretic position.

Immunofluorescence : Microscopic method of determining the presence or location of an antigen (or antibody) by demonstrating fluorescence when the preparation is exposed to a fluoresceintagged antibody (or antigen) using ultraviolet radiation.

Immunogen : A substance which is capable of eliciting (inducing) immune response. An immunogen usually has a fairly high molecular weight (usually greater than 10,000), thus, a variety of macromolecules such as proteins, lipoproteins, polysaccharides, some nucleic acids, and certain of the teichoic acids, can act as immunogens. Compare with antigen.

Immunoglobulin : Synonymous with antibody; five distinct classes have been isolated: IgG, IgM, IgA, IgE, and IgD.

Immunologic memory : The ability to rapidly produce large quantities of specific immune cells after subsequent exposure to a previously encountered antigen.

Immunology : The study of immunity and immune system.

Immunoperoxidase (IP) , assays are analogous to IFA in that antibody presence is identified on antigenic substrates visually. However, in the indirect IP instead of fluorescent dye-antibody conjugates, enzyme-antibody conjugated (principally peroxidase enzymes) are reacted with their corresponding substrates to produce a product which can be seen with a light microscope, eliminating the requirement for costly fluorescent microscopic equipment.

Immunoperoxidase stain : Combination of an enzyme that catalyzes production of colored product with an antibody to facilitate detection of certain antigens. particularly viral antigens.

Immunoradiometric Assay (IRMA) , uses low-level radioactively labeled specific antibody to quantitate low concentration compounds. In IRMA, a first antibody is presented on solid-phase (coated on tubes or beads). After binding the antigen present in the sample, a second radioactively labeled antibody is added. Radioactivity remaining after washing the solid phase is proportional to the concentration of antigen present in the sample and is quantitated by comparison to a standard curve.

Immunosuppression : Depression of the immune response caused by disease, irradiation, or administration of antimetabolites, antilymphocyte serum. or corticosteroids.

Impedance : Apparent resistance of a circuit to the flow of an alternating electric current.

Imperfect state : The asexual state or anamorph, usually conidia, of a fungus.

Impetigo : Acute inflammatory skin disease, caused by streptococci or staphylococci, characterized by vesicles and bullae that rupture and form yellow crusts.

In situ hybridization : Detection of nucleic acid of a pathogenic organism in tissue sections by separating the DNA into single-stranded molecules and allowing a labeled strand of homologous DNA to bind to the target. The target is visualized by developing the label (either enzymatic precipitate, fluorescence, or radiolabel).

In utero : Inside the womb.

In vitro : Literally means "in glass", away from a living organism; it is used to describe whatever happens in a test tube or other receptacle, as opposed to in vivo. When a study or an experiment is done outside the living organism, i.e., in test tube, it is said to be done in vitro.

In vivo : In the body, in a living organism, as opposed to in vitro; when a study or an experiment is done in the living organism, it is said to be done in vivo.

Inapparent infection : The presence of infection in a host without recognizable clinical signs or symptoms. Inapparent infections are identifiable only by laboratory means such as a blood test or by the development of positive reactivity to specific skin tests. (Synonym: asymptomatic, subclinical, occult infection).

Incidence : In reference to disease transmission, the number of cases of the disease in a specific subset of the population.

Incidence rate : The number of new cases of a specified disease diagnosed or reported during a defined period of time, divided by the number of persons in a state population in which the cases occurred. This is usually expressed as cases per 1,000 or 100,000 per annum.

Inclusion bodies : Microscopic bodies, usually within body cells; thought to be virus particles in morphogenesis.

Incubation period : Time span from introduction of disease-causing organisms until symptoms of the disease occur.

Indigenous flora or microflora : The mixture of microorganisms that typically reside on or in (colonize) various anatomic sites of a healthy animal body, such organisms may either be permanent or transient residents (In the past called normal or resident flora).

Indirect Fluorescent Antibody (IFA) , is the detection of antibodies to specific antigentic material in the substrate using fluorescent microscopy. Using fluorescently conjugated antibodies which are specific for a particular isotype of antibody, it is possible to distinguish IgG, IgA, and IgM isotypes of specific antibodies using IFA. This sensitive technique is highly specific in well-trained hands and recent developments in the establishment of internationally recognized standard materials have led to accurate quantitation of antibody concentrations through endpoint titration (the highest serial dilution of specimen at which specific fluorescence remains) and through measuring visual intensity of fluorescence compared to known reference standard material.

Indolent : Nearly or completely painless, sluggish, or inactive; used to describe a disease process.

Induced enzyme : An enzyme subject to induction.

Induced malaria : Malaria infection acquired by parenteral inoculation (e.g., blood transfusion or sharing of needles by drug addicts).

Induction : The process by which an enzyme is synthesized (induced) in response to the presence of an external substance, the inducer.

Induration : Abnormal hardness of a tissue or part resulting from hyperemia or inflammation, as in a reactive tuberculin skin test.

Infarct : Tissue death often due to an interruption in the blood supply to that tissue.

Infection : Invasion by and multiplication of microorganisms in body tissue resulting in disease. Infection is not synonymous with infectious disease; the result may be inapparent (see Inapparent infection) or manifest (see Infectious disease). The presence of living infectious agents on exterior surfaces of the body, or on articles of apparel or soiled articles, is not infection, but represents contamination of such surfaces and articles. (See Infestation and Contamination).

Infected individual : A person or animal that harbors an infectious agent and who has either manifest disease (see Patient or sick person) or inapparent infection (see Carrier). An infectious person or animal is one from whom the infectious agent can be naturally acquired.

Infection thread : In the formation of root nodules, a cellulosic tube through which Rhizobium cells can travel to reach and infect root cells.

Infectious agent : An organism (virus, rickettsia, bacteria, fungus, protozoan or helminth) that is capable of producing infection or infectious disease. Infectivity expresses the ability of the disease agent to enter, survive and multiply in the host; infectiousness indicates the relative ease with which a disease is transmitted to other hosts.

Infectious disease : A clinically manifest disease of humans or animals resulting from an infection. (See Infection).

Infestation : Presence of arthropods on the surface of the body or in the clothing; does not refer to endoparasites (Pediculus spp.). Infested articles or premises are those that harbor or give shelter to animal forms, especially arthropods and rodents.

Infiltrate : The invasion of the spaces in a tissue by materials not usually found in the tissue, such as tumors, infectious agents, white blood cells, etc.

Inflammation : A local protective response of the body; characterized by redness, pain, heat, and swelling.

Influenza : The disease caused by influenza viruses; also known as flu or grippe; influenza is an acute and highly infectious disease which occurs usually via the respiratory tract.

Influenza virus : The causative agent of influenza, includes influenza virus type A and B, but not influenza virus type C.

INH : Isoniazid.

Inh EIA : Inhibition enzyme immunoassay.

Inhibition : Prevention of growth or function.

Inhibitory quotient : Ratio of the average peak achievable level of antibiotic in a body fluid from which an organism was isolated to the MIC of that organism.

Inoculum : Material used to initiate a microbial culture.

Insecticide : Any chemical substance used for the destruction of insects, whether applied as powder, liquid, atomized liquid, aerosol or "paint" spray; residual action is usual. The term larvicide is generally used to designate insecticides applied specifically for destruction of immature stages of arthropods; adulticide or imagocide, to designate those applied to destroy mature or adult forms. The term insecticide is often used broadly to encompass substances for the destruction of all arthropods, but acaricide is more properly used for agents against ticks and mites. More specific terms such as lousicide and miticide are sometimes used.

Insertion : A genetic phenomenon in which a piece of DNA is inserted into the middle of a gene.

Insertion sequence (IS element) : The simplest type of transposable element. Has only genes involved in transposition.

Inspissation : Process of making a liquid or semisolid medium thick by evaporation or absorption of fluid.

Integration : The process by which a DNA molecule becomes incorporated into another genome.

Intercalary : Situated along the hypha, not at its end.

Interferon : A protein produced by cells as a result of virus infection which interferes with virus replication.

Intermediate host : Required host in the life cycle in which essential larval development must occur before a parasite is infective to its definitive host or to additional intermediate hosts.

Interspecies hydrogen transfer : The process in which organic matter is degraded anaerobically by the interaction of several groups of microorganisms in which hydrogen production and hydrogen consumption are closely coupled among species.

Interstitial : Spaces between the components of a tissue; e.g., in the lungs this pertains to spaces between the lung parenchyma.

Intertrigo : Erythematous skin eruption of adjacent skin parts.

Intracellular : Within cells.

Intracranial : Within the skull.

Intramuscular (intraperitoneal, intravenous) : Within the muscle (peritoneum, vein), as in intramuscular injection.

Intrapartum : During labor or delivery.

Intraperitoneally : Within the peritoneal cavity.

Intrauterine : Inside the cavity of the womb, as for an unborn child.

Intron : The intervening noncoding sequence in a split gene. Contrasted with exon, the coding sequence.

Intubation : The placement of a tubular device from the oro- or nasopharyngeal cavity into the trachea in order to assist with ventilation.

Invasiveness : Degree to which an organism is able to spread through the body from a focus of infection.

Ionophore : A compound which can cause the leakage of ions across membranes.

IP : Immunoperoxidase.

IRMA : Immunoradiometric Assay.

Ischemia : Cellular injury and death due to the impairment of blood flow to a region.

Isoenzyme (isozyme) : When two different enzymes catalyze the same reaction(s), they are isoenzymes to each other. Isoenzymes could differ from each other in their primary structure or electrophoretic mobility.

Isolation : As applied to patients, isolation represents separation, for the period of communicability, of infected persons or animals from others in such places and under such conditions as to prevent or limit the direct or indirect transmission of the infectious agent from those infected to those who are susceptible to infection or who may spread the agent to others. In contrast, quarantine (q.v.) applies to restrictions on the healthy contact of an infectious case.

Isotope : Different form of the same element containing the same number of protons and electrons, but differing in the number of neutrons.

Isozyme : See isoenzyme.

In vitro : Literally, within glass (i.e., in a test tube, culture plate, or other nonliving material).

In vivo : Within the living body.

Involution forms : Abnormally shaped bacterial cells occurring in an aging culture population.

Ion-exchange chromatography : Separation of components of a solution by chromatography based on the reversible exchange of ions in the solution with ions present in or on an external matrix.

Isotonic : Of the same osmolality of body tissues, red blood cells, bacteria, etc.

i.t. : Intrathecal.

IU : International unit(s).

IUD : Intrauterine device.

i.v. : Intravenous.

IVDU : Intravenoue drug use (user).

IVP : Intravenous pyelogram.