The human immunodeficiency virus' outer shell has a vulnerable spot that develops when a compound called glycan changes position, according to a study in the journal Nature Medicine. The finding increases the understanding of broadly neutralizing antibodies and could open the door to promising vaccine targets. The change in glycan's position enables an antibody to attach and inform the body that there is an invader in the system, said lead researcher Dr. Salim Abdool Karim, president of South Africa's Medical Research Council.