The impact of sex steroids, including androgens, oestrogens and progesterone (P4), on the activity of immune cells may contribute to sex differences and the effects of pregnancy on responses to 2009 H1N1. Generally, androgens, including dihydrotestoesterone and testosterone, suppress the activity of immune cells [124] . The immunosuppressive effects of androgens may reflect the inhibitory effects of androgen receptor signalling mechanisms on transcriptional factors that mediate the production of pro-inflammatory and antiviral cytokines [125] .