Secretory immunoglobulin A is a component of the human milk fat globule membrane

Secretory immunoglobulin A (sIgA), the predominant antibody fraction of human milk, represents a major protective factor against neonatal infection. Until now, sIgA had been identified only in the humoral fraction of human milk. The aim of this study was to assess whether in human milk sIgA is also associated with the milk fat globule (MFG). Using anti-sIgA-agglutinated human MFG and immune fluorescence microscopy, we could demonstrate that sIgA is, in fact, associated with human MFG. In human MFG membranes separated by Western blotting, sIgA bands were specifically stained, suggesting that sIgA is strongly associated with the human MFG membrane. This may be of physiologic relevance, inasmuch as earlier we could show that a quantity of undigested and functional human MFG are in fact found in the stools of the newborn. This would allow an additional extension of the protective mechanisms of sIgA throughout the whole intestine.