Review

The titer of neutralizing antibodies elicited in plasma or sera correlate closely with protection from infection for almost all human and veteri-nary viral vaccines where neutralization can be measured (Plotkin, 2008). Not only does this general observation underline the importance of the humoral arm of the immune response in vaccine design but it also highlights the crucial role of those antibodies that block infection at the cellular level. Most neutralizing antibodies act at the earliest steps in the viral replication cycle. They block interaction of the virus with receptors on the cell surface, prevent subse-quent conformational changes of viral proteins required for entry into cells, or transition from endocytic vesicles into the cytoplasm (Murphy et al., 2011). The human immunodeficiency viruses types 1 and 2 (HIV-1 and HIV-2) and related simian immunodeficiency viruses (SIVs) are not exceptions. Mutational escape from neutralization in infected individuals shows the relevance of neutralization in the natural history and course of HIV-1 infection (Deeks et al., 2006). Moreover, the passive transfer of neutral-izing antibodies can protect against subsequent challenge infection in nonhuman primate (NHP