| polymerase chain reaction | <molecular biology, technique> The first practical system for in vitro amplification of DNA and as such one of the most important recent developments in molecular biology. Two synthetic oligonucleotide primers, which are complementary to two regions of the target DNA (one for each strand) to be amplified, are added to the target DNA (that need not be pure), in the presence of excess deoxynucleotides and Taq polymerase, a heat stable DNA polymerase. In a series (typically 30) of temperature cycles, the target DNA is repeatedly denatured (around 90_C), annealed to the primers (typically at 50-60_C) and a daughter strand extended from the primers (72_C). As the daughter strands themselves act as templates for subsequent cycles, DNA fragments matching both primers are amplified exponentially, rather than linearly. The original DNA need thus be neither pure nor abundant and the polymerase chain reaction has accordingly become widely used not only in research, but in clinical diagnostics and forensic science. Acronym: PCR (14 Oct 1997) |
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| myelocytic leukemoid reaction | Leukocytosis of at least moderate degree, e.g., 50,000 or more per cu mm, with a few immature forms, e.g., 1 or 2% myelocytes, but chiefly mature polymorphonuclear leukocytes in the circulating blood; may be observed in association with tuberculosis, chronic osteomyelitis, various types of empyema, malaria, pneumococcal pneumonia, meningococcal meningitis, Hodgkin's disease, and metastases of carcinoma in the bone marrow. (05 Mar 2000) |
| coupled reaction | Two chemical reactions that share a common intermediate (for example, the productof the first reaction is a reactant in the second) and therefore havesome kind of energy exchange between them. (09 Oct 1997) |
| Porter-Silber reaction | The basis of the 17-hydroxycorticosteroid test; C-21 adrenocorticosteroids, which contain a dihydroxyacetone group at carbons 19, 20, and 21, react with phenylhydrazine. (05 Mar 2000) |
| Haber-Weiss reaction | The reaction of superoxide (O2-- with hydrogen peroxide to produce molecular oxygen (O2), hydroxide radical (OH-), and OH-; often, iron catalyzed; a source of oxidative stress in blood cells and various tissues. (05 Mar 2000) |
| cross reaction | <haematology, immunology> Serological reactions in which an antiserum against one antigen reacts with a non-identical but closely related antigen. (12 Dec 1998) |
| haemoclastic reaction | Haemolysis as observed in the laking of the blood. (05 Mar 2000) |
| cutaneous graft versus host reaction | An acute erythematous maculopapular reaction with bulla formation in the most severe cases; chronic changes may resemble lichen planus or scleroderma. (05 Mar 2000) |
| cutaneous reaction | The inflammatory reaction in the case of a skin test in a sensitive (allergic) subject. Synonym: cutaneous reaction. Origin: L. Cutis, skin, + reaction (05 Mar 2000) |
| Prausnitz-Kustner reaction | A test for the presence of immediate hypersensitivity in humans; test serum from an atopic individual is injected intradermally into a normal subject; the normal subject is challenged 24-48 hours later with the antigen suspected of causing the immediate hypersensitivity reaction in the atopic individual. Synonym: P-K test. (05 Mar 2000) |
| precipitin reaction | See: precipitin, precipitin test. (05 Mar 2000) |
| harlequin reaction | Sudden blanching of the lower half of the body of an infant lying on its side, leaving the remaining half of the body the normal pink colour. (05 Mar 2000) |
| cytotoxic reaction | An immunologic (allergic) reaction in which noncytotropic IgG or IgM antibody combines with specific antigen on cell surfaces; the resulting complex initiates the activation of complement which causes cell lysis or other damage, or which, in the absence of complement, may lead to phagocytosis or may enhance T lymphocyte involvement. (05 Mar 2000) |
| primary reaction | <dermatology> The cutaneous and sometimes systemic reactions associated with vaccination with smallpox vaccine. (18 Nov 1997) |
| Sakaguchi reaction | Guanidines in alkaline solution develop an intense red colour when treated with alpha-naphthol and sodium hypochlorite; a qualitative test for arginine, free or in a protein. (05 Mar 2000) |