Immunity is said to be passive when antibodies produced in other organisms are injected into a person to counteract antigens such as snake venom.
The yellowish fluid ‘colostrum’ secreted by the mother during the initial days of lactation is a form of natural passive immunity. It has plenty of antibodies (IgA) to protect the infant during the initial months of life. Maternal antibodies (IgG) are also transmitted through the placenta. Such an immunity is called natural passive immunity.