 To date, 24 platelet-specific alloantigens have been defined by immune sera, of which 12 are grouped in six biallelic systems (HPA-1, -2, -3, -4, -5, -15).For the remaining 12, alloantibodies against the thetical but not the antithetical antigen have been observed.The molecular basis of 22 of the 24 serologically defined antigens has been resolved.