Other Factors in Milk
In addition to antibodies, breast milk contains more than 100,000 constituents, which can influence the growth, development and immune status of the infant. Human colostrum is packed with living leukocytes (105–107 cells/ml) that enhance the immature infant's immune system. Usually leukocytes in breast milk are 40–50% macrophages, 40–50% polymorphonuclear neutrophils and 5–10% lymphocytes.
Antigen-specific memory B cells, plasmablasts and plasma cells in breast milk have been identified and characterized. Breast milk B cells are strikingly different from their blood counterpart: they mostly display a phenotype of IgD memory B cells and have a particular profile of adhesion molecules (CD44+, CD62L−, α4β7 +/− and α4β1 +), suggesting that these cells may originate from the gut-associated lymphoid tissue. Because of their low concentration, breast milk plasmablasts and plasma cells probably contribute only a small portion of antibodies secreted in milk.
Many cytokines (e.g., IL-1, IL-8, TGF-β and TNF), chemokines (e.g., MCP-1, MIP-1a and RANTES), growth factors (e.g., EGF, lactoferrin, cortisol, IGF, polyamines and nucleotides), hormones (e.g., erythropoietin, prolactin, insulin and leptin), vitamins (vitamin A, B1, B2, B6, B12 and D), neuropeptides (e.g., neurotensin and somatostatin), enzymes, mucins (MUC1 and MUC4) and antioxidants are present in milk at higher levels than those in the circulation. Generally speaking, there is considerable functional redundancy and many of these molecules act synergistically to compensate for the immunologic immaturity of the infant.
Future Virology. 2013;8(11):1127-1134. © 2013 Future Medicine Ltd.