HCV (and HIV) contain proteins with peptide sequences homologous to GPIIIa49-66. Infection with either (or both) of these viruses leads to presentation of these peptides to the host immune system and production of antibodies specific for GPIIIa49-66 by some individuals. The antibodies react with GPIIIa on autologous platelets, leading to platelet fragmentation and thrombocytopenia. Defective immune surveillance associated with HIV infection may explain breakage of immune tolerance to the autologous peptides. Professional illustration by Kenneth X. Probst.