An infant receives passive immunity through breast milk, which provides protection against gastrointestinal and respiratory pathogens. Which antibody isotype is predominantly responsible for this mucosal immunity in secretions like breast milk, tears, and saliva?

Explanation

Secretory IgA (sIgA) is the predominant antibody isotype found in external secretions such as breast milk, tears, saliva, and mucus. It plays a crucial role in mucosal immunity by preventing the attachment of pathogens to epithelial surfaces and neutralizing toxins, thereby providing a first line of defense at mucosal sites. IgG is the main antibody in serum and crosses the placenta, IgM is important in primary response, and IgE is involved in allergic reactions.