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What is Acute hemolytic transfusion reaction and explain in detail?
An acute hemolytic transfusion reaction (AHTR), also called immediate hemolytic transfusion reaction, is a life-threatening reaction to receiving a blood transfusion. AHTRs occur within 24 hours of the transfusion and can be triggered by a few milliliters of blood. The reaction is triggered by host antibodies destroying donor red blood cells. AHTR typically occurs when there is an ABO blood group incompatibility, and is most severe when type A donor blood is given to a type O recipient.

Signs and symptoms
Early acute hemolytic transfusion reactions are typically characterized by fever, which may be accompanied by rigors (chills). Mild cases are also typically characterized by abdominal, back, flank, or chest pain. More severe cases may be characterized by shortness of breath, low blood pressure, hemoglobinuria, and may progress to shock and disseminated intravascular coagulation. In anesthetized or unconscious patients, hematuria (blood in the urine) may be the first sign of AHTR. Other symptoms include nausea, vomiting, and wheezing.

Causes
The most common cause of acute hemolytic transfusion reaction is ABO incompatibility, which is typically due to human error that results in a recipient receiving the incorrect blood product. Rarely, other blood type incompatibilities can cause AHTR, the most common of which is Kidd antigen incompatibility. Rh, Kell, and Duffy antigen incompatibility have also been implicated in AHTR.

Mechanism
Antibodies against A and B blood groups (isohemagglutinins) present in the recipients blood destroy the donor red blood cells. They also activate the coagulation cascade (blood clotting system) via factor XII, which can lead to disseminated intravascular coagulation and kidney damage. Isohemagglutinins also activate the complement cascade via C3a and C5a, which then promote inflammatory cytokine release from white blood cells