The state of pregnancy represents an extreme challenge for the immune system. From the perspective of the pregnant female's immune system, the fetus is an allograft that contains foreign antigens from the father. To support a successful pregnancy, it is evolutionarily advantageous for a pregnant female's immune responses to shift away from inflammatory responses that contribute to fetal rejection and toward anti-inflammatory immune responses that aid in passive transfer of antibodies to the developing fetus (Raghupathy, 1997). Hormones contribute significantly to the shift in immune function that occurs over the three trimesters of pregnancy (Figure 1). Importantly, pregnant females are not immunosuppressed, but rather their immune responses are biased toward an anti-inflammatory phenotype that influences not only the outcome of pregnancy but disease pathogenesis as well.