Why are antibodies in breastmilk, specifically Immunoglobulin A (IgA), not degraded by stomach acid?

Why Antibodies in Breastmilk Resist Stomach Acid Degradation

Secretory IgA (sIgA) in breastmilk is specifically designed to resist degradation by stomach acid and digestive enzymes through its unique structural modifications, particularly the secretory component that protects it during passage through the infant's gastrointestinal tract. 1

Structural Protection Mechanisms

The Secretory Component Shield

• Dimeric IgA molecules in breastmilk are bound to a secretory component (part of the polymeric immunoglobulin receptor) that physically shields the antibody from proteolytic degradation 2
• This secretory component is attached during transcytosis through mammary gland epithelial cells, creating what is called secretory IgA (sIgA) 2
• The secretory component acts as a protective coating that makes sIgA inherently resistant to gastrointestinal juices and enzymes 1

Evidence of Stability During Digestion

• Studies in preterm infants demonstrate that anti-influenza A-specific IgA from mother's own breastmilk remains stable or even increases during gastric digestion, while other antibody types may be partially degraded 3
• Similarly, pertussis-specific IgA from mother's breastmilk survives intact through gastric digestion in preterm infants 4
• IgA rapidly increases in infant feces after breastfeeding begins, confirming its survival through the entire gastrointestinal tract 1

Clinical Significance

Passive Immune Protection

• The stability of sIgA allows it to provide continuous passive immunological protection throughout the digestive tract of the newborn infant 1
• This is particularly critical because neonates have immature immune systems and cannot produce adequate antibodies themselves 5
• Breastmilk antibodies play multifaceted roles including preventing infection, supporting commensal bacteria selection, and promoting tolerizing immunity during infancy 5

Comparison with Other Antibody Types

• While IgA demonstrates remarkable stability, IgG antibodies from breastmilk show variable resistance to gastric digestion depending on their specific antigen targets and maternal immunization history 3, 4
• IgM antibodies also show some resistance to digestion, though generally less studied than IgA 3
• The constant region and variable region of antibodies, along with maternal immunization status (previous infections and vaccinations), are critical factors determining their stability to proteolytic digestion 4

Important Caveats

Donor Milk Considerations

• Antibodies from donor breastmilk may be less stable during infant digestion compared to mother's own milk, particularly for IgA and IgG specific to certain pathogens 4
• Pasteurization of donor milk and differences in maternal immunization history affect antibody stability 4
• Anti-pertussis IgA and IgG from donor breastmilk decreased in gastric contents, while those from mother's own milk remained stable 4

Timing and Concentration

• sIgA levels are highest in colostrum during the first week of lactation, then decline but remain present throughout lactation 2
• The protective effect depends on continuous exposure through ongoing breastfeeding, as the antibodies provide passive rather than active immunity 1