In both preclinical and clinical trials, our HIV vaccines have been shown to induce both humoral (antibody) and cellular (T-cell) responses against HIV. The induction of both antibodies and T-cells is beneficial because these immune responses work through different mechanisms. Antibodies prevent infection by blocking viruses from infecting cells. In preclinical simian vaccine studies using repeated rectal challenges with moderate doses of virus, the avidity, or tightness, of antibody binding to the surface envelope glycoprotein of HIV correlates with the prevention of infection (The Journal of Infectious Diseases, 204:164 (2011)). In high dose challenges that infect all animals at the first exposure, the avidity of the antibody for envelope glycoprotein correlates with reduced levels of virus replication (Journal of Virology, 83:4102 (2009)). Similarly, antibody responses are believed to be critical for vaccine-elicited protection against Ebola and Marburg infection (Expert Review of Vaccines, 10:63 (2011)). These results likely reflect the tightly binding antibody both blocking infection as well as tagging virus and infected cells for destruction, by white blood cells such as macrophages, neutrophils and natural killer cells. Our vaccines elicit CD8+ T-cells, a type of T-cell that can recognize and kill cells that become infected by virus (without antibody tagging). For HIV, CD8+ T-cells are important for the control of the virus that has established an infection. For Ebola and Marburg, antibodies can stop or slow the progress of infection, but T cells are important for clearing the infection by killing remaining infected cells.