Final answer:
The transfer of antibodies from mother to fetus through the placenta is known as naturally acquired passive immunity. It provides the newborn with immediate protection against specific pathogens based on the mother's immune system's memory. This form of immunity is crucial for the infant's defense during the first few months of life.
Step-by-step explanation:
When antibodies are transferred from a mother to her fetus through the placenta, this is referred to as naturally acquired passive immunity. This type of immunity is crucial for the protection of the infant as it allows the transfer of maternal IgG antibodies, thus providing defense against infections. The primary mechanism behind this immune protection is that IgG, the only antibody class that can cross the placenta, is transferred from the mother's bloodstream directly into the fetal circulation. This passive transfer of antibodies grants the newborn immunological defense which can last up to six months post-birth.
In addition to the placental transfer, secretory IgA antibodies are also passed from mother to infant through breast milk. This further contributes to the infant's immune defense, especially against pathogens that they may encounter in their environment. The infant benefits from the immunological memory of the mother against specific pathogens she has been exposed to. While this form of immunity does not lead to the development of immunological memory in the infant, it provides immediate protection.
Passive immunity can also be administered medically through the injection of antibodies, like in the case of antivenom or therapeutic antibody serums. However, similar to natural passive immunity, the protection it offers is temporary as it does not elicit an active immune response from the person's own immune system.