Before revision: Due to the low density of envelope (Env) spikes on the surface of HIV-1, neutralizing IgG antibodies rarely bind bivalently using both antigen-binding arms (Fabs) to crosslink between spikes  , instead resorting to weaker monovalent binding that is more sensitive to Env mutations.
After revision: Due to the low density of envelope (Env) spikes on the surface of HIV-1, neutralizing IgG antibodies rarely bind bivalently using both antigen-binding arms (Fabs) to crosslink between spikes (inter-spike crosslinking) , instead resorting to weaker monovalent binding that is more sensitive to Env mutations.