An Acute Hemolytic Transfusion Reaction (AHTR) is a severe and potentially life-threatening complication that occurs when the recipient's immune system attacks transfused red blood cells, typically due to ABO blood group incompatibility. It can lead to symptoms such as fever, chills, back pain, and hemoglobinuria, and requires immediate medical intervention to prevent serious outcomes like acute renal failure or disseminated intravascular coagulation.