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Question - kiranraja
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08-03-2009, 02:02 PM
Which of the following could prevent an allergen from reacting with a specific IgE molecule present on the mast cell membrane?
A. Antihistamine B. Blocking antibody C. Cromolyn sodium D. Epinephrine E. Theophylline
08-03-2009, 02:41 PM
The correct answer is B. Blocking antibody is generally an IgG antibody against the allergen. It is induced in the allergic patient by administering small amounts of allergen over a period of time. When the person is again exposed to the allergen, the IgG reacts with the allergen before it can reach the IgE-coated mast cell. Antihistamines (choice A) would block histamine receptors but would not react with the allergen before it could reach the IgE-coated mast cell. Cromolyn sodium (choice C) is a drug that stabilizes mast cell membranes, thus inhibiting degranulation, but would have no effect on allergen binding to IgE on mast cells. Epinephrine (choice D) is the mainstay of therapy in severe cases of immediate hypersensitivity (anaphylaxis). It increases intracellular cyclic AMP, thus decreasing mast cell degranulation, and causes smooth muscle dilation in the airways. Epinephrine would have no effect on the binding of the allergen to IgE on mast cells. Cyclic AMP is degraded in cells by the enzyme phosphodiesterase. Theophylline (choice E) inhibits this enzyme, thereby increasing mast cell cyclic AMP levels and making degranulation less likely without affecting the binding of the allergen to IgE on mast cells. |
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