Allergic reactions are grouped into three categories depending on severity:
- mild or uncomplicated
- moderate or anaphylactoid
- life-threatening or anaphylactic reactions
Mild allergic reactions occur in about 1-3% of patients receiving blood products containing plasma. Symptoms are usually mild and include urticaria, erythema (skin redness), and itching. Hives can appear anywhere on the body and may vary in size. Symptoms usually occur within minutes after the start of the transfusion. They can often last for hours or even days. Mild allergic reactions result from a patient's hypersensitivity to soluble allergens in the plasma of the donor unit. Allergen substances may be drugs or food consumed by the blood donor.
Anaphylactoid and anaphylactic reactions have similar presentations. These reactions are rare but life-threatening. Anaphylactoid and anaphylactic reactions are severe systemic reactions with symptoms such as hypotension, dyspnea, nausea, vomiting, urticaria, and diarrhea. The most life-threatening symptoms include lower airway obstruction, laryngeal edema, cardiac arrhythmia, cardiac arrest, shock, and loss of consciousness.
None of these reactions present with fever.