Immune mechanisms leading to drug-induced blood dyscrasias.

Drug-induced blood dyscrasias may be due to toxicity of the drug, inborn errors of metabolism or immunological reactions. In the latter case, drug-induced antibodies are directed to specific target cells, such as platelets (thrombocytopenia) or granulocytes (agranulocytosis). Some of these immune mechanisms are based on the physical interaction between drug, antibody and target cells. A particular drug may bind to the surface of the blood cell which, as a carrier of the drug, will be destroyed as part of the immune reaction to the drug (hapten mechanism). Other drugs will form immune complexes with drug-specific antibodies. These immune complexes will bind to certain blood cells and destroy them as innocent bystanders (immune complex mechanism). In vitro assays are described which enable the offending drug to be detected by mimicking these immune mechanisms using patient serum, platelets or granulocytes, and the suspected drug. The diagnostic value of these in vitro assays is discussed.