Pathophysiology and diagnosis

Mr. J.R. has likely developed two separate transfusion reactions to the platelet concentrate he received.
The first reaction, manifested by the urticarial rash is likely a simple allergic reaction to the blood product. Allergic reactions are common and are mediated by the recognition of donor plasma antigens by preformed recipient IgE antibodies.
More severe allergic (anaphylactic) reactions can also occur. Symptoms of these reactions include hypotension and airway edema. These severe allergic reactions often occur when blood products are transfused to patients with IgA deficiency and who have antibodies directed against the IgA in these products.
The second reaction associated with an isolated fever is likely a febrile nonhemolytic transfusion reaction (FNHTR). A FNHTR is defined as an otherwise unexplained rise in temperature of at least 1 C during or after transfusion with a blood product. This type of reaction may be due to cytokines from the plasma of the donor or to recipient antibodies directed against antigens on the cells of the donor.