Mild allergic reactions result from a patient's hypersensitivity to soluble allergens in the plasma of the donor unit. The blood recipient forms antibodies to these allergens which are bound to IgE on mast cells. This causes the release of histamines. Histamines increase vascular dilation and permeability, which allows vascular fluids to escape into the tissues. Swelling occurs and itchy, raised, red welts appear. Allergen substances may be drugs or food consumed by the blood donor.
Anaphylactoid and anaphylactic reactions (collectively referred to as anaphylaxis) result from the recipient's forming anti-IgA, which targets IgA proteins in the donor plasma. Recipients have a genetic IgA deficiency. It is also believed that these types of reactions may be caused by other substances in the donor blood such as a peanut allergen transfused to a patient with a peanut allergy.