Colostral immune transfer

Stem cells

Colostrum provides stem cell transfer, from the maternal body and breast to the infant's body.

Immunoglobulins

Pregnancy, birth of the infant, and breastfeeding can be thought of as extreme physiological events which require fast adaptations of the maternal immune system.

The neonatal immune system remains undeveloped for about one month after the birth. Newborns can’t synthesize immunoglobulins during the first four weeks of life. In this time, infants rely largely on passive defenses of maternal origin in the form of antibodies provided

• During gestation by IgG placental transfer

• After birth through breastfeeding, which in particular provides a large amount of IgA antibodies.

Human colostrum is a rich source of immunoglobulins such as secretory IgA (SIgA) and, in smaller proportions, secretory IgM (SIgM), and IgG.

Secretory IgA (SIgA) in human milk comes in two different forms: IgA1 and IgA2, the latter being the more abundant in colostrum. These IgA subtypes bind diverse bacteria in the colostrum, which is hypothesised to help microbial adherence to the epithelial cells in the infant gut, and also with biofilm formation. IgA2 is found in higher concentrations in the large intestine and more resistant to bacterial proteases than IgA1.

• It has been speculated that plasma cells from mucosa-associated lymphatic tissue within the mammary gland are the source of IgA found in breastmilk, while IgG originates from blood.

• Overall, bovine colostrum contains on average three times more IgG-secreting white cells compared to IgA-secreting white cells.

S'anchez-Salguero et al 2025

The colostrum from mothers with a BMI > 30 kg/m2 contained significantly more IgG-secreting colostrum-resident B cells, more total IgG, and less total IgA. Lower colostrum IgA may explain increased vulnerability to infectious diseases reported for neonates of mothers living with obesity.

There are important extra-immune effects of maternal Ig, in addition to the classical protective effects of transferred maternal passive immunity, including effects on the development of the brain, gut, and possibly other organ systems in the neonate.

Leucocytes

Populations of leucocytes fall by four-fifths between colostrum and mature milk. The possible roles of ingested human milk leukocytes are unclear.

While it has been speculated that breastmilk cells are digested in the stomach, breastmilk leukocytes have been found in various organs of suckling pups, suggesting their ability to survive the digestion process and establish microchimerism, at least transiently.

A lactation-induced macrophage population has been described in the infant, which expands in situ and in response to the microbiota.

Colostral proteins

Whey proteins including alpha-lactalbumin, lactoferrin and secretory IgA are most highly concentrated in colostrum and reduce during the transition to mature milk.

Lactoferrin

Lactoferrin comprises about 6% of the total protein in colostrum. It is an iron-binding protein closely related to transferrin (which transports free iron in the body).

• Lactoferrin mostly destroys bacteria by binding free iron, needed by many bacteria and fungi to reproduce.

• Lactoferrin also has the ability to penetrate the cell walls of bacteria, which allows lysozyme to enter the bacterial cell, causing bacteria to lyse, or rupture.

Colostral lactose

Breast milk phase	Lactose concentration gm/l
Colostrum	40
Transitional	65
Mature	75

Increasing lactose levels pull in water by principles of solute diffusion.

You can find out about more about lactose and its volume stabilising function in human milk here.

Colostrum: microbiome transfer from maternal breast to newborn gut

The milk microbiota has a major role in the development of the infant’s immune system, influencing the health of the individual over the long term, including modulating the risk of allergies, diabetes, and infections. Human milk microbiome populations are diverse and complex, but found in low concentrations relative to other microbiomes in the body. A substantial proportion of the neonatal microbiota is derived from maternal milk.

Several dozens of bacterial genera and more than 200 species have been identified in colostrum. Proteobacteria and Firmicutes are the most abundant phyla in both colostrum and mature milk, with the keystone species (Serratia, Streptococcus and Staphylococcus) of breast milk closely interacting with human milk oligosaccharides.

The milk microbiome interacts with immunoglobulins and human milk oligosaccharides in a complex colostral ecosystem. For example,

• Immunoglobulins bind with and coat human milk bacteria, helping to initiate the infant gut microbiome.

• Human milk oligosaccharides adhere to bacteria.

Human milk oligosaccharides

Human milk as a synbiotic. That means that it comprises a mix of bacteria (which make milk probiotic) and human milk oligosaccharides (HMOs) (which are non-digestible prebiotics).

• Concentration of HMOs is highest in colostrum (average 9–22 g/L) and slightly lower in transitional milk (average 8–19 g/L from post-birth days 8 to day 15).

• As lactation progresses, the concentration of HMOs in mature breast milk gradually decreases, from 6 to 15 g/L in milk collected within 1 month to 4–6 g/L in milk collected within 6 months post birth.

You can find out more about HMOs here.

Colostral fats

There is substantially less fat in colostrum than in mature human milk.

Overall concentrations of lipids in breast milk have been shown to correlate with maternal serum lipid levels, peaking at two to four hours after a meal. For example, a study by Nascimentoa et al 2025 showed that colostrum fat concentrations increased by 20% two hours after lunch.

The fat concentration of human milk directly relates to maternal body fat reserves, local synthesis by the mammary gland, amounts of lipid in the plasma, and maternal food intakes.

You can find out more about fats in human milk here.

Possums Breastfeeding & Lactation Foundations

How to hand express your colostrum or milk

Does it help to express colostrum before the birth and when would you use it?

Colostrum: innoculating your newborn with living immune tissue

Possums Breastfeeding & Lactation Advanced

Colostrum: evolutionary origins, mechanisms of secretion, and biomarkers of secretory activation

Thinking about colostrum through a reductionist lens could worsen breastfeeding outcomes

Colostrum: key ingredients

Selected references

Ge H, Zhu W, Zhang J. Human milk microbiota and oligosaccharides in colostrum and mature milk: comparison and correlation. Frontiers in Nutrition. 2024:DOI 10.3389/fnut.2024.1512700.

Nascimentoa R, Hochmana V, De Silvaa C. Immediate effects of food intake by the nursing mother on the macronutrient content of colostrum. Jornal de Pediatria. 2025;20(30).

S'anchez-Salguero E, Prieto-Ch'avez JL, Garcia-Alonso CAe. Maternal obesity associates with altered humoral immunity in blood and colostrum. Mucosal Immunology. 2025;18:491-499.