Final answer:
The immunoglobulin that confers natural passive immunity to breastfed infants through mucous membrane secretions is secretory IgA.
Step-by-step explanation:
The immunoglobulin found in mucous membrane secretions and that provides natural passive immunity for breastfed infants is secretory IgA (Immunoglobulin A). IgA is present in mucous secretions like saliva and tears, and most importantly for newborns, in breast milk, known as colostrum. It is crucial for providing passive immunity to infants as their own immune systems are still developing. Secretory IgA exists as a dimer, which allows it to be effective in trapping and neutralizing pathogens within mucous secretions. Newborns receive this protective antibody through breastfeeding, which confers protection against various infectious agents.