| SDS | same day surgery; school dental services; self-rating depression scale; sensory deprivation syndrome... |
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| SF | Sabin-Feldman [test]; safety factor; salt-free; scarlet fever; screen film; seminal fluid; serosal f... |
| SFP | screen filtration pressure; simultaneous foveal perception; spinal fluid pressure; stopped flow pres... |
| SFR | screen filtration resistance; stroke with full recovery |
| SR | sarcoplasmic reticulum; saturation recovery; scanning radiometer; screen; secretion rate; sedimentat... |
| natural disasters | Sudden calamitous events producing great material damage, loss, and distress. They are the result of natural phenomena such as earthquakes, floods, etc. (12 Dec 1998) |
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| natural dyes | Dye's obtained from animals or plants; examples include carmine, obtained from cochineal in the dried female insect Dactylopius cacti of Central America, and haematoxylin, extracted from the bark of the logwood tree Haematoxylon campechianum in the Caribbean area. (05 Mar 2000) |
| natural focus of infection | An ecosystem in which an infectious agent normally persists in nature; e.g., yellow fever virus in a jungle monkey-Haemagogus mosquito ecosystem. (05 Mar 2000) |
| natural haemolysin | Haemolysin occurring in the plasma of an animal of one species, e.g., a dog, which fixes complement with the red blood cells of some other species, e.g., a rabbit, thereby causing haemolysis of the cells of the rabbit, although the dog was not previously exposed to antigenic stimulation with such cells. (05 Mar 2000) |
| natural history | A former branch of knowledge embracing the study, description, and classification of natural objects (as animals, plants, and minerals) and thus including the modern sciences of zoology, botany, and mineralogy insofar as they existed at that time. In the 17th, 18th, and 19th centuries it was much used for the generalised pursuit of certain areas of science. (webster, 3d ed; from dr. James h. Cassedy, nlm history of medicine division) (12 Dec 1998) |
| natural immunity | This includes local barriers to infection such as skin, stomach acid, mucous, the cough reflex, enzymes in tears and saliva and skin oils. (27 Sep 1997) |
| natural killer cell | <haematology, immunology> A certain type of white blood cell that attacks tumour cells and infected body cells by binding to a tumour cell and releasing lethal chemicals. Natural killer cells are thought to play a key role in cancer prevention by killing abnormal cells before they multiply and grow. (16 Dec 1997) |
| natural killer cells | Large granular lymphocytes which do not express markers of either T or B-cell lineage. These cells do possess Fc receptors for IgG and can kill target cells using antibody-dependent cell-mediated cytotoxicity. NK cells can also use perforin to kill cells in the absence of antibody. Killing may occur without previous sensitization. Synonym: NK cells. (05 Mar 2000) |
| natural killer cell stimulating factor | <cytokine> A 75 kD heterodimeric cytokine composed of disulfide-bonded 40 kD and 35 kD subunits that was originally identified by its ability to induce cytotoxic effector cells in synergy with less than optimal concentrations of interleukin-2. It is released by macrophages in response to infection and promotes the activation of cell-mediated immunity. Specifically, IL-12 triggers the maturation of Thl CD4 cells, specific cytotoxic T-lymphocyte responses and an increase in the activity of NK cells and consequently, it is the initiator of cell-mediated immunity. It enhances the lytic activity of NK cells, induces interferon production, stimulates the proliferation of activated T-cells and NK cells. Is secreted by human B lymphoblastoid cells (NC 37). May play a role in controlling immunoglobulin isotype selection as it also inhibits IgE synthesis (even in the presence of anti-IFN monoclonal antibody) and as a growth factor for activated CD4+ and CD8+ T-cells independently of interleukin-2, and for CD56+ NK cells but not resting peripheral blood mononuclear cells or resting or activated tonsillar B-cells. It is likely that interleukin 12 has a major role in protective immunity against viruses and is under study as an immunotherapy in HIV infection. Formerly referred to as cytotoxic lymphocyte maturation factor. Acronym: IL-12 (12 Dec 1998) |
| natural language processing | Computer processing of a language with rules that reflect and describe current usage rather than prescribed usage. (12 Dec 1998) |
| natural mutation | A mutation which occurs by itself without first being affected by a mutagen, for example during the process of DNA replication. Spontaneous mutations arise at a remarkably constant rate. The rate that spontaneous mutations arise has been used as an evolutionary clock to estimate how closely related two (or more) separate species are to each other. (09 Oct 1997) |
| natural passive immunity | Immunity conferred by the mother on the foetus or newborn. (14 Nov 1997) |
| natural pigment | A naturally occurring coloured compound; absorbs light in the visible range of the electromagnetic spectrum. Compare: structural colour. Synonym: biochrome. (05 Mar 2000) |
| natural products | Naturally occurring compounds that are end products of secondary metabolism; often, they are unique compounds for particular organisms or classes of organisms. (05 Mar 2000) |
| natural selection | The hypothesis that genotype environment interactions occurring at the phenotypic level lead to differential reproductive success of individuals and hence to modification of the gene pool of a population. (18 Nov 1997) |