If all these anti-bodies do is block access to the receptor, then how are they different from existing receptor antagonists?
For instance, for heroin addicts, wouldn't this be the same as implanting some sort of device that released a steady stream of a very strong opiate antagonist like naloxone into the body? These treatments exist, I believe, but I'm not sure how well they work in practice. I can't imagine a heroin addict voluntarily signing up for such an approach, which would basically amount to starting an irreversible process of withdrawal, with its attendant and various miseries. I suppose you could coerce people or give them this sort of option over jail-time, but it seems pretty cruel to force somebody to go through withdrawal of this sort, especially with alternatives like methadone and buprenorphine available.
In the case of cocaine, where there is not the same sort of withdrawal problem but the receptors targeted are those for dopamine, other problems seem like they would present themselves. For instance, I would imagine that antagonism of dopamine receptors leads to all sorts of unpleasant side effects... I'd guess severe depression, but also possibly very serious muscular side effects like those associated with Parkinson's disease. To do this with the immune system (which could presumably go haywire and do permanent damage that administering the drug with a pill or even an implant could not) seems pretty dangerous. How do you know that a couple years down the road after you've administered this thing that makes the body antagonize its dopamine receptors that people will still be able to walk and have voluntary control of their muscles.
The general idea (of antagonizing the receptor site for drugs of abuse) is not new but it has not been particularly successful. I know there is, for instance, an alcohol inhibitor <link> that was never developed and marketed because of problems with half-life, but also with serious problems of side-effects of its use. With all of these receptor antagonists it seems that (especially in addicts) you get effects paradoxical to the drug of abuse. Taking a drug addict and not just blocking the effect of their drug of choice but also blasting them into a realm where they are feeling displeasure relatively equal and opposite to the pleasure of their drug of choice does not seem like a viable long term strategy in getting people to stop using drugs. Sure, you could mandate the use of such treatments, but then again you could just lock people up in solitary confinement; in either case the result will be that they are not getting high anymore. The problem arises when you want that person to rejoin society and be able to make the choice to remain drug free.
In addition to this fundamental problem with receptor antagonist strategies, as articulated above, I would worry a great deal about serious, permanent side effects of using the immune system to modulate the nervous system. It sounds like a great way to induce a serious, irreversible, massively unpleasant cascade of side effects.