Final answer:
IgD in human cord blood supports antigen recognition and the maturation of B cells. While IgG antibodies crossing the placenta provide passive immunity to the developing fetus, IgD indicates early immune system development.
Step-by-step explanation:
Immunoglobulin D (IgD) found in human cord blood primarily supports the process of antigen recognition and the maturation of B cells into plasma cells. B-cell receptors (BCRs) of the IgD class are present on the surface of naive B cells and play a pivotal role in initiating the immune response. Although IgG is the antibody class known for crossing the placenta and providing protection to the fetus, the presence of IgD in the cord blood is indicative of the early development of the immune system in the newborn.
It's important to note that the presence of IgG antibodies is significant in providing passive immunity to the developing fetus, as IgG can cross the placental barrier from the mother's blood to the fetal blood supply. This transfer is a crucial aspect of the immune defense for the infant, which can last up to six months after birth.