Another oestrogen that affects the functioning of the immune system is oestriol (E3), which is produced during pregnancy by the placenta. When E3 levels are high, inflammatory responses and the symptoms of Th1mediated autoimmune diseases -including multiple sclerosis -are reduced [127, 128] . Whether the effects of pregnancy on responses to 2009 H1N1 reflect the effects of E3 on immune responses requires investigation.