Significance The immune system normally produces antibodies against pathogens and avoids making antibodies against self-proteins. In some individuals, antibodies against self-proteins (autoantibodies) are made and can cause debilitating disease, but the reasons for this failure of self-tolerance are not known. The experiments described test the following hypothesis linking viral infections and production of autoantibodies: A B cell whose immunoglobulin receptor recognizes a self-membrane protein can capture that protein from the membrane of a virus-infected cell and simultaneously cocapture viral proteins; fragments of the viral protein can then be presented to antiviral T cells and qualify for the positive signals for proliferation and antibody production known as T-cell help. We observed this phenomenon in vitro and in vivo.