Secretory immunoglobulin A (sIgA), the predominant antibody fraction of hum
an milk, represents a major protective factor against neonatal infection. U
ntil now, sIgA had been identified only in the humoral fraction of human mi
lk. 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 hu
man 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 sI
gA is strongly associated with the human MFG membrane. This may be of physi
ologic relevance, inasmuch as earlier we could show that a quantity of undi
gested and functional human MFG are in fact found in the stools of the newb
orn. This would allow an additional extension of the protective mechanisms
of sIgA throughout the whole intestine.