... the cocaine-antibody complex is too large to cross the blood-brain barrier. If the cocaine were weakly bound, it would spend part of its time detached from

... the cocaine-antibody complex is too large to cross the blood-brain barrier. If the cocaine were weakly bound, it would spend part of its time detached from the antibodies, and during those times be able to enter the brain in the usual way. Since the antibodies have a large effect on the psychological effect-response curve, the binding must be quite strong. Therefore cocaine is sequestered throughout the body.