COMEDK 2007 MCQ Answer 30

1. Artificially induced in laboratory.

2. Induced localized inflammatory skin reaction in previously sensitized rabbit (to horse serum) by intradermal injection of cognate Antigen.

Inflammation grossly visible after several hours

a. Longer time than Type I, but shorter than Type IV. Classified as immediate because responsible antibodies can be passively transferred via serum.

b. Ag-Ab complexes form and adhere to vascular endothelium; complement system activated and some of its chemotactic intermediates attract neutrophils to site. Anaphylatoxins degranulate mast cells with resultant release of histamine that causes constriction of arterioles and retards blood supply to area. Platelets stimulated by the immune complex initiate the coagulation cascade, resulting in fibrin deposits. Eventually vessels become clogged with thrombin and accumulated cells, causing an exudate into the surrounding tissue (edema). Deprivation of blood supply to area results in ischemic necrosis.

c. Only precipitating (multivalent) antibodies can elicit the Arthus reaction (mainly IgG). Relatively large amounts of antigen required.