Natural infection with many viruses results in the production of antibodies which help clear the virus and protect us from subsequent reinfection. Eliciting such a response is the basis of many effective vaccines, but unfortunately little is understood about protective antibody responses in HIV, which has hindered HIV vaccine design. Despite sharing many similarities with HIV-1, most patients with HIV-2 do not develop AIDS (although a minority do) and the reasons for this are not entirely clear. This project proposes to compare neutralizing antibody responses in HIV-1 and HIV-2 infection and explore whether stronger responses are found in HIV-2 infected patients who do not progress to AIDS, when compared to HIV-2 progressors and HIV-1 patients.||Early studies also suggested that HIV-2 antibodies could render HIV-1 non-functional and although some previous studies claimed HIV-2 infected individuals may be protected against subsequent HIV-1 infection, the majority suggest no protection or even an increased risk of acquiring HIV-1 superinfection. We therefore propose to compare HIV-1 cross-neutralizing antibody responses and enhancing antibody responses in HIV-2 patients who go on to acquire HIV-1 superinfection, with those who have remained HIV-2 mono-infected despite possible exposure to HIV-1.||Such information could provide vital clues to how the HIV surface interacts with antibodies and the importance of eliciting an antibody response in future HIV vaccines.