hypersensitivity

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hypersensitivity
1. (in immunology) Increased or abnormal sensitivity to compounds, which can elicit a specific immune response accompanied by tissue damage. Hypersensitive reactions include allergies and anaphylaxis. See also hapten.

2. (in botany) A reaction by the tissues of a plant in response to invasion by pathogenic viruses, bacteria, fungi, or other organisms. The reaction varies according to the type of infecting agent, but typically involves the synthesis of enzymes (such as chitinase) that disarm or degrade the pathogen, the production of phytoalexins, which inhibit the growth of pathogens, and strengthening of cell walls in the vicinity of the attack by deposition of lignin and other materials. The final strategy is programmed cell death (apoptosis) around the infection site, which slows the spread of the pathogen to neighbouring healthy tissues. See also elicitor; systemic acquired resistance.

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hypersensitivity Condition in which a person reacts excessively to a stimulus. Most hypersensitive reactions are synonymous with an allergy, the commonest being hay fever.

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