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What is Allergic transfusion reaction and explain in detail?
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.

Signs and symptoms
Cause
Allergic reactions from blood transfusion may occur from the presence of allergy-causing antigens within the donors 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:

Diagnosis
An allergic transfusion reaction is diagnosed if two or more of the following occur within 4 hours of cessation of transfusion:

A probable diagnosis results if any one of the following occurring within 4 hours of cessation of transfusion:
The UK hemovigilance reporting system (SHOT), has classified allergic reactions in to mild, moderate and severe. Reactions can occur that have features of both allergic and febrile reactions.

Mild
A rash, urticaria, or flushing

Moderate
Wheeze (bronchospasm) or angioedema but blood pressure normal and no respiratory compromise