Hypersensitivity reactions to heparin: delayed onset thrombocytopenia and necrotizing skin lesions.

Significant hypersensitivity reactions to heparin may manifest as either delayed onset thrombocytopenia or as necrotizing skin lesions. Such hypersensitivity reactions are uncommonly recognized, partly because of their rarity and partly because their existence is not widely appreciated. Both adverse reactions may be readily diagnosed. In the presence of these reactions, continuing heparin therapy may lead to serious thrombo-embolic complications and death of the patient. In this study, two patients who highlight the clinical settings of these reactions are discussed with reference to their pathogenesis and clinical significance.