An abnormal, immune-mediated fall in the number of circulating platelets, caused by a recipient’s reaction to foreign antigens on platelets he or she has received during a transfusion. Although the immune reaction normally starts against the donated platelets, in PTP host (recipient) platelets are also attacked, leading to a severe decrease in platelet numbers about a week, plus or minus 2 days, after the platelet transfusion. The consequences of a low platelet count may include bleeding, bruising, or discoloration of the skin.