There are two models for this: the hapten model and the autoantibody model. The hapten model proposes that certain drugs, especially penicillin and cephalosporins, will bind to certain proteins on the red cell membrane and act as haptens (small molecules that can elicit an immune response only when attached to a large carrier such as a protein; the carrier may be one that also does not elicit an immune response by itself). Antibodies are created against the protein-drug complex, leading to the destructive sequence described above. The autoantibody model proposes that, through a mechanism not yet understood, certain drugs will cause antibodies to be made against red blood cells which again leads to the same destructive sequence.