| reversible reaction | A chemical reaction that takes place in either direction i.e., from the forward or reverse direction; ionization is such a reaction, as are reaction's involving racemases, isomerases, mutases, transferases, etc. (05 Mar 2000) |
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| graft-versus-host reaction | <haematology> A common and serious, complication of bone marrow transplantation where there is a reaction of donated bone marrow against a patient's own tissue. When donor lymphocytes or a graft containing lymphocytes that are immunologically competent are given to a patient that has low immunological competence, an incompatibility reaction can result. This is due to antibodies from the donor against antigens in the host. This is due to mismatch of MHC Class I antigens and can produce lymphocyte clones that will react by a variety of processes against the host and cause damage. The clinical condition can be fatal and is due to the donor's immune cells recognising the host cells as foreign. The clinical entity characterised by anorexia, diarrhoea, loss of hair, leukopenia, thrombocytopenia, growth retardation, and eventual death brought about by the graft-versus-host reaction. It can occur in either chronic or acute forms and is treatable by immunosuppressive drugs. Seen most commonly following bone marrow transplantation, acute disease is seen after 5-40 days and chronic disease weeks to months after transplantation, affecting, principally, the gastrointestinal tract, liver, and skin. Radiological appearances of the gastrointestinal tract include; thickened wall, mucosal folds thickened or effaced, increased secretions most likely to be rapid transit of GI tract, mass most likely to be focal oedema, fibrosis, hallmark: diffuse, uniform thickening of small bowel. Synonym: GVH disease. Acronym: GVHD (20 Sep 2002) |
| chemical reaction | <chemistry> A process in which one or more substances are changed chemically into one or more different substances. (09 Oct 1997) |
| periodic acid Schiff reaction | <technique> A histochemical technique based on periodic acid oxidation of a substance containing the 1,2-glycol grouping. It is used for staining carbohydrates as the resulting dialdehyde reacts with Schiff reagent to form a coloured product. The adjacent hydroxyl groups are oxidized to form aldehydes by periodic acid (HIO4) and these aldehyde groups react with Schiff's reagent (basic fuchsin decolourised by sulphurous acid) to give a purple colour. Acronym: PAS (18 Nov 1997) |
| periosteal reaction | <radiology> In adults: hypertrophic osteoarthropathy (HOA), pachydermoperiostosis, vascular insufficiency, thyroid acropachy, fluorosis in kids: physiologic periostosis, Caffey disease, JRA, acute leukaemia, scurvy, rickets, congenital syphilis, hypervitaminosis A (12 Dec 1998) |
| periosteal reaction patterns | <radiology> Pattern most indicative of buttressing eosinophilic granuloma laminated Ewing sarcoma or hair-on-end Codman triangle osteo- or chondrosarcoma or sunburst (per Resnick) (12 Dec 1998) |
| peroxidase reaction | Formation of indophenol blue by the action of an oxidizing enzyme present in certain cells and tissues when they are treated with a solution of alpha-naphthol and dimethylparaphenylenediamine; by this method, cells of the myelocyte series, which give a positive reaction, may be distinguished from those of the lymphocyte series, which give a negative reaction; endothelial leukocytes give a variable reaction, probably positive when they have phagocytised the debris of myeloid cells. Synonym: Nadi reaction. (05 Mar 2000) |
| group reaction | A reaction with an agglutinin or other antibody that is common (though usually in varying concentrations) to an entire group of related bacteria, e.g., the coli group. (05 Mar 2000) |
| Gruber's reaction | Agglutination reaction as applied to the diagnosis of typhoid. Synonym: Gruber's reaction, Gruber-Widal reaction. (05 Mar 2000) |
| Gruber-Widal reaction | Agglutination reaction as applied to the diagnosis of typhoid. Synonym: Gruber's reaction, Gruber-Widal reaction. (05 Mar 2000) |
| cholera-red reaction | A test for cholera vibrio whereby the addition of 3 or 4 drops of sulfuric acid (concentrated, chemically pure) to an 18-hour-old bouillon or peptone culture of the organism produces a colour from rose-pink to claret. (05 Mar 2000) |
| Gunning's reaction | The formation of iodoform from acetone by iodine and ammonia in alcohol. (05 Mar 2000) |
| chromaffin reaction | Production of a yellow-brown to brown colouration in normal and abnormal cells containing epinephrine and norepinephrine, when fresh tissue slices are placed in a dichromate-chromate mixture overnight; useful for detection of pheochromocytoma (adrenal medulla) and other tumours which produce catecholamines. (05 Mar 2000) |
| circular reaction | In sensorimotor theory, the tendency of an organism to repeat novel experiences. (05 Mar 2000) |
| phosphoroclastic reaction | Cleavage of C-C bonds that involves phosphate transfer but not, as in phosphorolysis, directly to one of the products; e.g., the decomposition of pyruvate to acetate + CO2, in which Pi is added to ADP to form ATP. (05 Mar 2000) |