An allergic transfusion reaction is when a blood transfusion results in allergic reaction. It is among the most common transfusion reactions to occur. Reported rates depend on the degree of active surveillance versus passing reporting to the blood bank. Overall, they are estimated to complicate up to 3% of all transfusions. The incidence of allergic transfusion reactions is associated with the amount of plasma in the product. More than 90% of these reactions occur during transfusion. An allergic transfusion reaction is when a blood transfusion results in allergic reaction. It is among the most common transfusion reactions to occur. Reported rates depend on the degree of active surveillance versus passing reporting to the blood bank. Overall, they are estimated to complicate up to 3% of all transfusions. The incidence of allergic transfusion reactions is associated with the amount of plasma in the product. More than 90% of these reactions occur during transfusion. Allergic reactions from blood transfusion may occur from the presence of allergy-causing antigens within the donor's blood, or transfusion of antibodies from a donor who has allergies, followed by antigen exposure. An allergic transfusion reaction is a type of transfusion reaction that is defined according to the Center for Disease Control (CDC) as: An allergic transfusion reaction is diagnosed if two or more of the following occur within 4 hours of cessation of transfusion: