Clinical hematology is the field of medicine most closely related to immunology, since pathologically altered cells and other components of the immune response circulating in the blood form the basis of some hematological diseases. The author presents a female patient with acute lymphoblastic leukemia, another female patient suffering from autoimmune hemolytic anemia, immune thrombocytopenia and pernicious anemia, and a male patient treated for aplastic anemia. Disorders characterized by malignant proliferation of cells involved in the immune response include acute lymphoblastic leukemia, malignant lymphomas and malignant immunoproliferative diseases. A pathologically altered immune response is associated with the appearance of antibodies which may be directed against erythrocytes, platelets, neutrophils, coagulation factors, hemopoietic progenitor cells, parietal cells of the gastric mucosa or the intrinsic factor. The detection and, in particular, confirmation of an immune disorder often require the use of so-called immunological tests, which are based on individual reactions of the immune response. For the diagnosis of leukemias, malignant lymphomas and, in particular, subtypes of acute lymphoblastic leukemia, immunophenotyping of surface and cytoplasmic immunoglobulins is performed using flow cytometry. One of the oldest immunological investigations is the Coombs test for confirmation of autoimmune hemolytic anemia. Immune disorders are treated more or less successfully with various immunosuppressive agents, such as corticosteroids, antilymphocyte globulin and cyclosporin. Only good knowledge of immune disorders and the methods used for their diagnosis can guarantee reliable prediction of the course of illness and its successful management.