Allergic drug reactions mediators

The most common clinical picture of non-immediate RCM reactions is a macu-lopapular exanthema, which resembles other drug-induced T-cell-mediated hypersensitivity reactions. The reported onset of skin eruptions 2-10 days after the first exposure to a RCM and 1-2 days after re-exposure to the same substance is typical for an allergic drug reaction with a sensitization phase. 

Drug reactions mediated by immune responses can have several different mechanisms. Thus, any of the four major types of hypersensitivity discussed earlier in this chapter (page 967) can be associated with allergic drug reactions ... 

Co-administration of beta-blockers has been associated with an increased risk of severe allergic drug reactions and reduces the effect of adrenaline in the immediate treatment of anaphylactic shock. The mechanism involves changes in the regulation of anaphylactic mediators (281). 

Allergic drug reactions can involve most of the major components of the immune system, including the cellular elements, immunoglobulins, complement, and cytokines. Most immunoglobulin isotypes have been imphcated in immunologically mediated drug reactions. 

Cyproheptadine has antianaphylactic activity that is associated with its ability to slow down the release of histamine and other mediators from fat cells. It is mainly used for treating bronchial asthma attacks, allergic bronchitis, rhinitis, and allergic skin reactions as well as in adjuvant therapy for anaphylactic reactions. Synonyms of this drug are periactin and vimicon. 

In some drug reactions, several of these hypersensitivity responses may present simultaneously. Some adverse reactions to drugs may be mistakenly classified as allergic or immune when they are actually genetic deficiency states or are idiosyncratic and not mediated by immune mechanisms (eg, hemolysis due to primaquine in glucose-6-phosphate dehydrogenase deficiency, or aplastic anemia caused by chloramphenicol). 

Allergic responses to drugs are mediated by the release of histamine or histamine-like substances, and they commonly present as skin rashes, particularly urticaria. More serious hypersensitivity responses include bronchospasm or the acute, explosive anaphylactic reaction with cyanosis and cardiovascular collapse. A delayed reaction known as serum sickness, although more often associated with such drugs as the penicillins and cephalosporins rather than with serum, manifests clinically 7 to 10 days after receiving the drug or serum as fever, malaise, joint pains, and urticarial skin rashes. 

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