Pseudoallergic Reactions

These are effects that mimic allergic reactions but have no immunological basis and are largely genetically determined. They are due to release of endogenous, biologically active substances, e.g. histamine and leukotrienes, by the drug. A variety of mechanisms is probably involved, direct and indirect, including complement activation leading to formation of polypeptides that affect mast cells, as in true immunological reactions. Some drugs may produce both allergic and Pseudoallergie reactions.

Pseudoallergie effects mimicking type I reactions (above) are called anaphylactoid and they occur with aspirin and other nonsteroidal anti-inflammatory drugs (indirect action as above) (see also Pulmonary reactions, above); corticotrophin (direct histamine release); i.v. anaesthetics and a variety of other drugs i.v. (morphine, tubocurarine, dextran, radiographic contrast media) and inhaled (cromoglicate). Severe cases are treated as for true allergic anaphylactic shock (above) from which, at the time, they are not distinguishable.

Type II reactions are mimicked by the haemolysis induced by drugs (some antimalarials, sulpho-namides and oxidising agents) and food (broad beans) in subjects with inherited abnormalities of erythrocyte enzymes or haemoglobin (see p. 123).

Type III reactions are mimicked by nitrofurantoin (pneumonitis) and penicillamine (nephropathy). Lupus erythematosus due to drugs (procainamide, isoniazid, phenytoin) may be Pseudoallergie.

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