Britannica Sources used in this answer

How does IgG provide immune protection to a developing fetus?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

Diagram of an antibody molecule showing two identical light chains and two identical heavy chains joined by disulfide bonds in a Y-shaped structure, with labeled regions for antigen binding and effector function. — antibody The four-chain structure of an antibody, or immunoglobulin, molecule. The basic unit is composed of two identical light (L) chains and two identical heavy (H) chains, which are held together by disulfide bonds to form a flexible Y shape. Each chain is composed of a variable (V) region and a constant (C) region.

In humans, IgG is the only class of immunoglobulin capable of crossing the placenta to provide immune protection to a developing fetus.12

Throughout the final two-thirds of pregnancy, maternal IgG antibodies are actively transported from the mother’s bloodstream into the fetal circulation. This passive transfer ensures that the newborn is born with a sample of the mother's antibodies, providing temporary protection against microbial invaders to which the mother has previously developed immunity.1

While this mechanism is vital for neonatal health, it is temporary, as these antibodies gradually undergo metabolic breakdown and become diluted as the infant grows. Additionally, because IgG is the only class that crosses the placenta, the fetus does not receive other types of antibodies, such as IgA, until after birth through colostrum and breast milk.12