CD8A and respiratory system disorder: Although CD4+ T-cell responses were overall lower in frequency and number than CD8+ T-cell responses, all vaccination regimens elicited CD4+ T-cell responses of a Th1-type response, even in the Th2-biased BALB/c background, a response indicating that the potential for antibody-dependent enhancement and subsequent enhancement of respiratory disease caused by Th2-type lung immunopathology is reduced19–21.