A 35-year-old male patient presents with numerous subcutaneous hemorrhages. History and physical examination reveal that he has been taking sedormid (a sedative) for the past week. Laboratory tests indicate normal hemoglobin and white blood cell levels with significant thrombocytopenia (very low platelet count). You suspect that he has developed a drug-induced type II hypersensitivity reaction. This reaction may occur if the drug does which of the following?

Haptens (incomplete antigens) are not themselves antigenic, but when coupled to a cell or carrier protein become antigenic and induce antibodies that can bind the hapten alone (in the absence of the carrier protein). They are small molecules that are generally less than 1000 kDa. While haptens react with antibodies, they are not immunogenic because they do not activate T cells and cannot bind the MHC. Haptens are significant in disease; penicillin is a hapten and can cause severe life-threatening allergic reaction by destruction of erythrocytes. Catechols in the oils of poison ivy plants are haptens and cause a significant skin inflammatory response. Chloramphenicol is a hapten that can lead to the destruction of leukocytes and cause agranulocytosis. Sedormid is a hapten that can cause thrombocytopenia and purpura (bleeding) through the destruction of platelets.