Defensins: Learn About the Antibiotics in Milk

Breast milk is nature's strategy for humans to face the challenge of survival. Defensins contained in breast milk are associated with protection against diarrheal diseases, respiratory tract infections, and necrotizing enterocolitis. 
Defensins: Learn About the Antibiotics in Milk
Luz Eduviges Thomas-Romero

Written and verified by biochemist Luz Eduviges Thomas-Romero.

Last update: 27 December, 2022

Defensins are a huge family of small proteins (or peptides) that exert different defense strategies against pathogenic microorganisms. These peptides are now known to be encoded by an ancient and diverse gene family.

Defensins are present in most multicellular organisms. These include everything from plants, fungi, insects, mollusks, arachnids, to even mammals, including humans. During their evolution, defensins have become very diversified and acquired new functions in different species.

Moreover, defensins were able to evolve to be highly efficient in their antimicrobial responses to a wide range of pathogens.

Defensins are part of the innate defense that living beings possess

This family of antimicrobial peptides is part of the innate immune response that all living beings share. There are fundamental differences between prokaryotic cells (for example, bacteria) and eukaryotic cells (those in our body), which represent attack targets for defensins.

Defensins: Learn About the Antibiotics in Milk

Various studies were able to show that defensins kill a wide range of microorganisms. These include bacteria, enveloped viruses, fungi, and even transformed or cancerous cells.

Why are peptides multitasking?

Unlike most conventional antibiotics, defensins have the ability to function as immunomodulators. This is how these antimicrobial peptides become true “secret weapons” of nature.

Specifically, these peptides are capable of enhancing phagocytosis, stimulating the release of prostaglandins, and neutralizing the septic effects of LPS (a bacterial component). Furthermore, they can promote the recruitment and accumulation of various immune cells in inflammatory areas. They can also increase angiogenesis (revascularization) and induce wound repair.

It’s easy to see that these antimicrobial peptides constitute one of the most researched alternatives to conventional antibiotics. Due to their variety of functions, defensins have broad prospects for clinical applications.

Which tissues produce defensins?

They mainly occur in epithelia. Thus, they can appear on the surface of the intestine, lungs, and skin, and also in the mammary gland. In these environments, defensins confer protection directly against bacterial colonization of the epithelium. Defensins are also present in leukocyte granules, saliva, and oral epithelium.

Does breast milk contain defensins?

Yes. In fact, the defensin concentrations reported in human milk exceed those reported on other mucosal surfaces. To date, only two families of antimicrobial peptides have been identified in human milk: the cathelicidins and the defensins.

We’ve been aware of the bactericidal, antiviral, and antifungal properties of breast milk for a long time. Accordingly, the incidence of illnesses such as diarrhea or respiratory infections is significantly lower in infants who breastfeed than in infants that drink formula.

Thus, antimicrobial peptides are believed to contribute strongly to the lower incidence of necrotizing enterocolitis, respiratory tract infections, and other gastrointestinal diseases in infants that breastfeed.

Defensins: Learn About the Antibiotics in Milk

Defensin content is tailored to the baby’s needs

In preterm births, there is a higher susceptibility to systemic infections and necrotizing enterocolitis (NEC). There are reports that indicate higher concentrations of alpha defensins, both in colostrum and in mature milk, of mothers who gave birth prematurely. This is put in comparison to colostrum and mature milk from mothers of full-term babies.

Additionally, we know that the beta defensins in human milk inhibit the growth of two bacterial strains, which have a connection with NEC (Salmonella and E. Coli), suggesting their role in reducing the risk of NEC.

An example of teamwork

It’s a collective belief that a variety of protective factors in human milk are responsible for the protective effect of breastfeeding. In this fluid, the baby’s defense system consists of immunoglobulins, which provide specific protection. Nonetheless, antimicrobial proteins are important in this system. The most relevant are defensins, lysozyme, lactoperoxidase, and lactoferrin.

They also protect the mammary glands

The production and secretion of defensins by the epithelium of the mammary gland is greater in colostrum than in mature milk. At present, we consider defensins to have a protective function for the mother and prevent mastitis.

All cited sources were thoroughly reviewed by our team to ensure their quality, reliability, currency, and validity. The bibliography of this article was considered reliable and of academic or scientific accuracy.

  • Reddy, K. V. R., Yedery, R. D., & Aranha, C. (2004). Antimicrobial peptides: premises and promises. International journal of antimicrobial agents, 24(6), 536-547.
  • Lei, J., Sun, L., Huang, S., Zhu, C., Li, P., He, J., … & He, Q. (2019). The antimicrobial peptides and their potential clinical applications. American journal of translational research, 11(7), 3919.
  • Lopez-Exposito, I., & Recio, I. (2008). Protective effect of milk peptides: antibacterial and antitumor properties. In Bioactive components of milk (pp. 271-294). Springer, New York, NY.
  • Baricelli, J., Rocafull, M. A., Vázquez, D., Bastidas, B., Báez-Ramirez, E., & Thomas, L. E. (2015). ß-defensin-2 in breast milk displays a broad antimicrobial activity against pathogenic bacteria. Jornal de pediatria, 91(1), 36-43.
  • Lewis, E. D., Richard, C., Larsen, B. M., & Field, C. J. (2017). The importance of human milk for immunity in preterm infants. Clinics in perinatology, 44(1), 23-47.

This text is provided for informational purposes only and does not replace consultation with a professional. If in doubt, consult your specialist.