The extraordinary mutability and resulting genetic diversity of HIV, which is substantially more complex than that of other human viruses, also present a formidable obstacle to immune control. By the time the body produces antibodies directed at the outer HIV envelope protein, which is the key target for neutralizing antibodies, the protein has mutated
in such a way that the circulating antibodies cannot neutralize it. New antibodies are induced, but new mutations repeatedly enable the virus to evade the immune system. Furthermore, although broadly neutralizing antibodies could
persist in the host and potentially neutralize the virus even as it mutates, these are rarely found in vivo and are apparently difficult to induce, since their epitopes tend to be conformationally masked and not readily accessible for immune recognition and response.2