Hypersensitivity Reactions
المؤلف:
APURBA S. SASTRY , SANDHYA BHAT
المصدر:
Essentials Of Medical Microbiology 2021
الجزء والصفحة:
3rd edition , p193-194
2025-09-15
372
Definition
The term hypersensitivity or allergy refers to the injurious consequences in the sensitized host, following subsequent contact with specific antigens.
Gell and Coombs Classification
Following an antigen contact, hypersensitivity may occur immediately or after a few days. It may result from abnormality of either humoral or cell-mediated immune response. Based on the above two features, Gell and R Coombs classified hypersensitivity reactions into four types (Table 1).
Table1. Features of various types of hypersensitivity reactions.
Immediate Hypersensitivity Reactions
These reactions occur immediately, within few minutes to few hours of antigen contact, as a result of abnormal exaggerated humoral response (antibody mediated). This can be further classified into three types based on the type of effector mechanisms:
1. Type I hypersensitivity reaction: It is IgE-mediated, which causes mast cell degranulation following a contact with soluble antigen
2. Type II hypersensitivity reaction: It is IgG (or rarely IgM) mediated, which causes complement activation or antibody-dependent cellular cytotoxicity (ADCC) in response to cell surface bound antigens
3. Type III hypersensitivity reaction: It is immune complex-mediated; which are formed due to interaction between soluble antigen and antibody (usually IgG), resulting in an abnormal inflammatory response.
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