Fetal hypoxygenation due to the anemia may be an important factor in stimulating the maturation processes of the immune system in the fetus, determining the switch from fetal to adult form. We performed an immunological evaluation in a fetus in the 29th week of gestation with secondary hypoxemia and RhD and C alloimmune anemia. We found normal relative values for total T, TCD4+ and TCD8+, and low levels of B memory and NK cells. NKT cells were absent. There IL-8 serum levels were higher than those of the mother. Fetal anemia was not capable of activating the humoral immune response and differentiation of memory B cells. Our data suggest that NKT cell differentiation may be related to thymus development in humans.