ABSTRACT
Usingadifferentapproach,Smithetal.(48)cametosimilarconclusions.Theantibody domaincontrollingreactionsbetweenplateletmembranesanddrug-dependentantibodiesfrom patientswiththrombocytopeniainducedbycinchonaalkaloidswasstudiedusingF(ab'h,Fab, andFefragmentsmadefrompurifieddrug-dependentantibody.Bydirectbindingradioimmunoassay(RIA)measurements,20,000--50,000antibodymoleculesboundperplateletequivalent ofpurifiedplateletmembranesatapparentsaturationwiththreedifferentantibodies.F(ab')z andFabfragmentsboundtoplateletmembranesdrugdependentlybutFefragmentsdidnot. Theabilityofdrug-dependentIgGfragmentstocompetewithintactIgGwasquantitatively measuredbyRIAandbycomplementfixation.F(ab'handFabcompetedwithintactlgGat an8:1and>50:1molarratio,respectively,inRIA,andata1.6-3:1and44-75:1ratio, respectively,bycomplementfixationassays.FedidnotcompetewithIgGineitherassay. Smithetal.(48)concludedthattheFabdomainsupportedattachmentofdrug-dependent antibodytotheplateletmembrane.
