ABO incompatibility.

Antibodies bound to red cells in the body can cause red cell destruction by two major mechanisms: intravascular and extravascular.

Intravascular:

Red cells are destroyed in the bloodstream with consequent release of haemoglobin into the circulation. The antibodies that can cause this type of reaction, that is, IgM or IgG anti-A or anti-B, cause rapid activation of the complement cascade usually by the classical pathway.

Extravascular:

Intact red cells are removed from the circulation by cells of the mononuclear phagocyte system situated in the liver and spleen. Red cells coated with IgG or sensitised with complement to the C3 stage, but which do not proceed through the cascade to the C5b-C9 lytic complex, may interact with mononuclear phagocytes, notably the macrophage. Extravascular red cell destruction results in breakdown products of haemoglobin, such as bilirubin and urobilinogen, in the plasma and urine. This type of red cell destruction can be caused by IgG, anti-D and other rhesus antibodies.

Non-immune haemolytic transfusion reactions occur when red blood cells (RBCs) are damaged before transfusion resulting in haemoglobinaemia and haemoglobinuria without significant clinical symptoms.

Non-haemolytic febrile transfusion reactions are usually caused by cytokines from leukocytes in transfused red cell or platelet components, causing fever, chills, or rigors. In the setting of transfusion administration, a fever is defined as a temperature elevation of 1°C.

Patients with hereditary C1-inhibitor deficiency may have recurrent attacks of angioedema when transfused with standard plasma containing blood components.