Certain drugs that are themselves harmless sometimes trigger immune responses that can have serious consequences for the patient. Two categories are distinguished: the hypersensitivity reactions and the autoimmune reactions.
Autoimmune reactions occur when immune cells attack their own cells. Examples include autoimmune hepatitis (halothane), lupus-like syndromes (isoniazide, hydralazine), and haemolytic anaemia caused by methyldopa.
There are four types of hypersensitivity reactions.