Which immunoglobulin class can cross the placenta from mother to fetus?

The immunoglobulin class that can cross the placenta from mother to fetus is IgG. This transfer is crucial for providing the fetus with antibodies, which help to protect against infections during the early months of life. IgG is the most abundant antibody in serum and is specifically designed to pass through biological barriers. The ability of IgG to achieve this transfer is largely attributed to the presence of a specific receptor on placental cells, which facilitates the transport of IgG across the placental membrane.

In contrast, IgM is too large in size to effectively cross the placenta, as it is typically found as a pentamer in its functional form. IgA is primarily found in mucosal areas, such as the gut and respiratory tract, and while it plays a significant role in mucosal immunity, it does not cross the placenta. IgE, while important for allergic responses and defense against parasites, also does not cross the placenta in significant amounts. Thus, IgG stands out as the immunoglobulin class that is effectively transported to the fetus, providing crucial maternal immunity during pregnancy.