Figure 1.

Leukocyte development begins in the yolk sac before moving to the liver and finally the bone marrow. The development and maturation of primary lymphoid organs (blue) and peripheral blood leukocytes (red) occur throughout gestation but is not complete until after birth. The light gray shading shows the gestational ages of preterm births from the threshold of viability (24 weeks; at which 50% of infants in developed countries survive) to 37 weeks of gestation. Based on a figure from Durandy (2003).