The invention relates to the adhesion of platelet GpIba to strand b3 of domain A1 of von Willebrand factor (vWF), said strand b3 comprising amino acid residues at amino acid position 560-566 and/or a functional part or equivalent thereof, said platelet GpIbalpha, said GpIbalpha region comprising an amino acid sequence corresponding to a beta-switch loop of platelet GpIbalpha, comprising amino acid residues at amino acid position 227-242 and/or a functional part or equivalent thereof. The invention provides a method to interfere with said adhesion of blood platelets to vWF comprising modulating said adhesion. The invention further provides proteinaceous compounds, antibodies, medicaments and pharmaceutical compositions to that end. The invention also provides means and methods to increase platelet adhesion by topical application of a compound increasing platelet adhesion.