| reaction of partial identity | See: gel diffusion precipitin tests in two dimensions. (05 Mar 2000) |
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| reaction quotient | <chemistry> A constant represented by the symbol Q which is defined as the product of the concentrations of the products, each raised to the power that corresponds to its coefficient in the balanced equation, divided by the product of the concentrations of reactants, each raised to the power that corresponds to its coefficient in the balanced equation (see the equation in the equations page). at equilibrium conditions Q=K, the equilibrium constant. (09 Jan 1998) |
| reaction time | The time from the onset of a stimulus until the organism responds. (12 Dec 1998) |
| reaction, allergic | A reaction that occurs when the immune system attacks a usually harmless substance (an allergen) that gains access to the body. The immune system calls upon a protective substance called immunoglobulin e (IgE) to fight these invading allergic substances ( allergens). Even though everyone has some IgE, an allergic person has an unusually large army of these IgE defenders -in fact, too many for their own good. This army of IgE antibodies attacks and engages the invading army of allergic substances of allergens. As is often the case in war, innocent bystanders are affected by this battle. These innocent bystanders are special cells called mast cells. These cells are frequently injured during the warring of the IgE antibodies and the allergic substances. When a mast cell is injured, it releases a variety of strong chemicals including histamine into the tissues and blood that frequently cause allergic reactions. These chemicals are very irritating and cause itching, swelling, and fluid leaking from cells. These allergic chemicals can cause muscle spasm and can lead to lung and throat tightening as is found in asthma and loss of voice. (12 Dec 1998) |
| reactivate | 1. To render active again. 2. In particular, of an inactivated immune serum to which normal serum (complement) is added. (05 Mar 2000) |
| reactivation | The restoration of activity to something that has been inactivated. (18 Nov 1997) |
| reactive arthritis | Reiter's syndrome is also called reactive arthritis since it is thought to involve the immune system which is reacting to the presence of bacterial infections in the genital, urinary, or gastrointestinal systems. Accordingly, certain people's immune systems are genetically primed to react aberrantly when these areas are exposed to certain bacteria. The aberrant reaction of the immune system leads to inflammation in the joints and eyes. (12 Dec 1998) |
| reactive astrocyte | <cell biology> A round to oval astrocyte cell with abundant cytoplasm containing glial filaments and an eccentric nucleus; may contain two nuclei in the cell hypertrophy of astrocytes. Synonym: gemistocyte, gemistocytic cell, reactive astrocyte, reactive cell. (05 Mar 2000) |
| reactive attachment disorder | Markedly disturbed and developmentally inappropriate social relatedness that begins before age 5 and is associated with grossly pathological child care. The child may persistently fail to initiate and respond to social interactions in a developmentally appropriate way (inhibited type) or there may be a pattern of diffuse attachments with nondiscriminate sociability (disinhibited type). (12 Dec 1998) |
| reactive cell | <cell biology> A round to oval astrocyte cell with abundant cytoplasm containing glial filaments and an eccentric nucleus; may contain two nuclei in the cell hypertrophy of astrocytes. Synonym: gemistocyte, gemistocytic cell, reactive astrocyte, reactive cell. (05 Mar 2000) |
| reactive depression | A psychological state occasioned directly by an intensely sad external situation (frequently loss of a loved person), relieved by the removal of the external situation (e.g., reunion with a loved person). (05 Mar 2000) |
| reactive hyperaemia | Hyperaemia following the arrest and subsequent restoration of the blood supply to a part. (05 Mar 2000) |
| reactive inhibition | Tendency toward a lessened strength of response due to practice or activity. It is independent of the effect of reward and is a direct function of time interval since the last response and the number of preceding responses. (12 Dec 1998) |
| reactive oxygen species | Reactive intermediate oxygen species including both radicals and non-radicals. These substances are constantly formed in the human body and have been shown to kill bacteria and inactivate proteins, and have been implicated in a number of diseases. Scientific data exist that link the reactive oxygen species produced by inflammatory phagocytes to cancer development. (12 Dec 1998) |
| reactive perforating collagenosis | A rare skin disorder characterised by extrusion of collagen fibres through the epidermis; usually begins in infancy or childhood and appears clinically as recurrent umbilicated papules that resolve spontaneously. The condition may be inherited or acquired, the latter differing from Kyrle's disease because follicular involvement is absent. (05 Mar 2000) |