Posttransfusion Purpura

Pathophysiology

PTP is an immune thrombocytopenia resulting from platelet alloantibodies, most often anti-HPA-1a alone or in combination with antibodies to other platelet antigens almost invariably on the GPIIb/IIIa receptor complex. The reaction is usually due to transfusion of RBCs, but other products (e.g., platelets and plasma) have been implicated. The transfused product contains the immunogenic platelet glycoprotein, which induces an anamnestic response. PTP occurs most frequently in women who were previously sensitized during pregnancy, yet sensitization from transfusion does occur. About 2% of the general population is homozygous HPA-1b; however, the incidence of PTP is lower than that would be expected given this frequency. In 2015, 305 cases were reported in the United States to the National Blood Collection and Utilization Survey (NBCUS), at a rate of 1:57,823 components transfused. This represents an increase from 2013 (259 cases or 1:78,014 components transfused) and a further increase from 2011 (209 cases or 1:100,158 components transfused). Patients who develop anti-HPA-1a often share certain human leukocyte antigen (HLA) genotypes, such as HLA-B8 or HLA-DRB3∗0101, similar to that seen in neonatal alloimmune thrombocytopenia (NAIT). Interestingly, the increased risk of PTP in women who have previously had children with NAIT has not been reported. Women with a history of PTP may be at increased risk for having a pregnancy affected by NAIT. The antibody involved in PTP destroys both transfused and autologous platelets. The mechanism of destruction of autologous platelets is unknown; theories include the following:

• The antibody produced cross-reacts with autologous platelets.

• Donor-derived, soluble platelet glycoprotein is adsorbed on to the autologous platelets.

• The immune response includes an autoimmune component.