The yellowish fluid "colostrum" secreted by mammary glands of...

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.