Immunohematologic disorders are a broad group of entities in which hematologic diseases, usually cytopenias, are caused by immune reactions. These reactions lead to the development of hemolytic anemia, thrombocytopenia, or neutropenia, either separately or in combination. Common underlying mechanisms include immunodeficiency, systemic autoimmunity, and drug-induced reactions. Transplacental transfer of maternal alloantibodies can lead to fetal and neonatal cytopenias. Other immune reactions include antibodies to clotting factors, which result in both thrombosis and hemorrhage, and hemolytic and nonhemolytic reactions during transfusion of blood products.