A proposed model for DDAb binding to an epitope on a platelet glycoprotein. (Left) Antibodies capable of causing drug-dependent thrombocytopenia react weakly with an epitope on a glycoprotein. The K_a for this interaction is too small to allow significant numbers of antibody molecules to bind in the absence of drug. (Right) Drug contains structural elements that are complementary to charged or hydrophobic domains (H) on the GP epitope and the complementarity-determining region (CDR) of the antibody. Drug interacts with the target protein and antibody to improve the “fit” between the 2 proteins, increasing the K_a to a value that permits binding to occur at levels of antibody, antigen, and drug achieved in the circulation after ingestion of the drug.