The article presents a study that shows that the HIV-1 negative factor (Nef) protein crosses into bystander B cells and blocks the production of class-switched immunoglobulins such as IgG and IgA. B cells generate IgG and IgA by a process known as immunoglobulin class switch recombination. However, the inhibition of immunoglobulin class switching by HIV-I Nef prevents the production of antibody classes that are most skillful at neutralizing the virus.