But up to now, the development of anti-drug antibodies has been associated only with worse efficacy - not (to my knowledge at least) with any sort of severe safety issue. This was a very unusual case indeed - I don't recall seeing another example of a death claimed to be from anti-drug antibodies (as distinct from some sort of anaphylaxis).
Clinical relevance of serum natalizumab concentration and anti-natalizumab antibodies in multiple sclerosis