What type of immunity is defined by maternal antibodies being passed to a child?

Natural passive immunity refers to the transfer of antibodies from one individual to another without requiring the recipient's immune system to produce those antibodies. In the context of maternal antibodies being passed to a child, this occurs through the placenta during pregnancy and also through breast milk after birth. This type of immunity provides the infant with immediate protection against pathogens that the mother is already immune to, leveraging the mother's immune experience.

Natural passive immunity is considered "natural" because it involves the direct transfer of antibodies from a natural source (the mother) to the child, rather than through medical or artificial means. This is different from artificial passive immunity, where antibodies are provided through medical interventions, such as an injection of antibodies from another source.

The other immunity types mentioned do not accurately describe this process. Natural active immunity involves the individual generating their own antibodies in response to exposure to an infection, while artificial active immunity refers to the creation of antibodies in response to a vaccine.