Important events that occur in an immune-mediated hemolytic transfusion reaction (HTR) include:
- Antibody binding to red blood cells
- Antibodies may be either IgM or IgG.
- IgM antibodies activate complement and lead to intravascular hemolysis where free hemoglobin is released into the plasma.
- IgG antibodies rarely activate complement but they are often involved in effecting phagocytosis.
- The concentration of the antibody is directly related to the severity of the HTR.
- Activation of complement
- The end result of complement activation is red cell lysis.
- Activation of mononuclear phagocytes and cytokines
- Sensitized red cells are removed from circulation by mononuclear phagocytes. Macrophages in the spleen and Kupffner cells in the liver are active in this process.
- Activation of coagulation
- Antibody-antigen complexes may initiate coagulation and cause disseminated intravascular coagulation (DIC).
- Shock and Renal Failure
Hemolysis can be intravascular or extravascular. In intravascular hemolysis, free hemoglobin, RBC stroma, and intracellular enzymes are released into the blood stream. This results in hemoglobinemia and hemoglobinuria, which can lead to kidney damage. In extravascular hemolysis, there is no release of free hemoglobin. Sensitized red cells are removed from the circulation by the monocytes and macrophages in the reticuloendothelial system.