Strong antibodies hold on to activate GPIb-IX signaling. (A) Antibodies sometimes bind to GPIbα on 2 different platelets. When the platelets move at different speeds, these GPIb-antibody complexes are pulled. (B) A strong antibody holds on tightly whereas the intraplatelet forces unfold the stalk of GPIbα. This transmits a signal inside the cell, likely by allowing the other subunits in the GPIb-IX complex to move closer together. (C) A weak antibody will let go of GPIbα before the stalk unfolds.