The following signs and symptoms are associated with acute HTR due to ABO incompatibility but can be associated with other blood group incompatibilities. ABO incompatibility typically results from patient misidentification.

The more serious symptoms result from intravascular hemolysis (IVH) caused by antibodies such as anti-A and anti-B that can bind complement to C9.

Signs and symptoms typically appear within minutes of the transfusion but can occur anytime during the transfusion. They may include:

1. Burning sensation along the vein being transfused (IVH due to complement activation to C9)
*2. Lower back pain in the area of the kidneys (renal failure with subsequent oliguria/anuria)
*3. Unexplained bleeding/oozing from a surgical site (fibrinolysis following DIC)
*4. Hypotension leading to hypovolemic shock (release of vasoactive substances caused by C3a and C5a)
5. Tightness in substernal area of the chest (bronchial constriction due to release of vasoactive substances caused by C3a and C5a fragments)
6. Other symptoms: fever, chills, skin flushing, dyspnea, wheezing, anxiety, malaise, nausea, headache.

* If untreated, these complications may lead to patient death.