A model for DDAb binding. DDAbs of the type induced by quinine and other drugs are weakly autoreactive with a glycoprotein (in this case GPIIb/IIIa) expressed on platelets but the K_A for this interaction is too small to be of clinical consequence. When the drug is present, it binds to antibody CDR and induces a structural change that significantly increases K_A. When binding occurs, the drug is trapped at the antibody-antigen interface. Under conditions where bivalent antibody binding is possible, the effective K_A is increased by orders of magnitude, leading to accumulation of sufficient antibody on the platelet surface to promote platelet clearance.