In December 2025, the National Academies of Sciences, Engineering, and Medicine published a report titled: “Breastfeeding in the United States: Strategies to Support Families and Achieve National Goals.” This report provides a framework for helping families meet their breastfeeding goals, with a focus on the health benefits of breastfeeding to both mother and baby, of which there are many.
While the physical and emotional benefits of breastfeeding are incredible enough in their own right, the immunological benefits are perhaps the most fascinating. In this article, we’ll discuss how breastfeeding shapes your baby’s immune system—and the role kissing your baby can aid in that process (as if you needed another reason to do that!).
Physical health benefits of breastfeeding
Critically, the National Academies of Sciences report states that:
“Human milk provides all the nutrients necessary to support optimal growth and development for nearly all infants for the first six months of life and provides continued essential nutrition and developmental benefits for the duration of breastfeeding… infants who are breastfed ‘more’ versus ‘less’ may have a reduced risk of developing noncommunicable diseases such as asthma, otitis media, obesity in childhood, and childhood leukemia…
“In addition, a protective association of breastfeeding has been found for infant mortality, including sudden infant death, rapid weight gain and growth, systolic blood pressure, severe respiratory and gastrointestinal infections in younger children, allergic rhinitis, malocclusion, inflammatory bowel disease, and type 1 diabetes…”
For mothers, breastfeeding may also be associated with “lower rates of breast cancer, ovarian cancer, hypertension, and type 2 diabetes” [1].
Emotional health benefits
In addition to the physical health benefits it provides, breastfeeding can also be an emotional boon for both moms and babies. The act of breastfeeding promotes bonding between mom and baby, which helps babies to establish a sense of secure attachment, reinforces the voice bond that began in the womb, and provides comfort and a sense of continuity for babies from pre- to post-birth [2].
Immunological benefits
Breastfeeding clearly provides babies with an overall foundation of emotional and physical wellness. But did you know that a mother’s breast milk can change in response to pathogens she picks up from her infant, providing important immunological benefits as well? [3]
Newborns are born with immature immune systems, and in order to help their little bodies to fight off illness, a mother’s milk will produce antibodies in response to pathogens that either her infant carries, or that she was exposed to through other means [4]. These antibodies help to provide immunity to infants until their immune systems are more fully developed.
Breastmilk antibody production
There are multiple immunoglobulins, or antibodies, that help infants’ bodies to recognize and attack foreign invaders, or antigens, like bacteria and viruses. The most predominant types of antibodies found in breast milk include Immunoglobulin G (IgG), Immunoglobulin M (IgM), and Immunoglobulin A (IgA).
All three of these antibodies play a role in infant immunity and work together to provide a perfectly choreographed dance of protection against antigens. While there are proteins in breast milk, like lactoferrin, which also help to prevent infections, let’s examine how each of the antibodies in breast milk work to protect vulnerable bodies [5].
The most predominant types of antibodies found in breast milk include Immunoglobulin G (IgG), Immunoglobulin M (IgM), and Immunoglobulin A (IgA). All three of these antibodies play a role in infant immunity and work together to provide a perfectly choreographed dance of protection against antigens.
Immunoglobulin G
Immunoglobulin G offers protection through two different mechanisms. First, when a pregnant mother is prenatally exposed to pathogens, B lymphocytes (types of white blood cells made in the bone marrow) then turn into plasma cells that create antibodies. These antibodies then pass through the placenta in increasing numbers in the third trimester, and enter the fetal bloodstream, which protect the infant for the first six-twelve months of life as his or her own immune system gradually becomes capable of producing its own IgG [7], [8], [9].
Second, IgG is transported via blood into the mammary gland and expressed into breast milk. While placental IgG provides systemic immunity, or the protection of blood and tissues throughout the body, this is not quite the case with breast milk IgG. Unlike IgA, IgG is only produced in trace amounts in breast milk, with consistent ranges from around 0.1-1mg/mL throughout lactation, to levels ranging from 10 to 100 mg/mL in colostrum and levels around 1 mg/mL in mature milk for IgA [10].
IgG in colostrum and mature milk is mainly for local gut protection (the digestive tract from the mouth to the anus) to prevent damage or infection. IgG in the infant gut targets and neutralizes toxins, viruses, and bacteria, while helping to train the infant’s developing immune system [11]. Infants must rely on their own production of IgG for systemic protection that starts producing at around 3-6 months.
Immunoglobulin M
Immunoglobulin M starts production in the baby’s liver prenatally at around 20 weeks, and is the first antibody that infants produce after birth [12]. IgM is therefore the baby’s first line of defense against infections, helping to “buy time” systemically and in the digestive tract while IgG and IgA begin protection, and continues as placental IgG protection wanes.
Immunoglobulin A
Last, but not least, is the star of the infant antigen-fighting show: Immunoglobulin A. IgA is the antigen that’s most present in breastmilk, though it does work in tandem with IgG and IgM to support digestive immunity. In fact, a recent study found that “IgA levels are the highest in colostrum… representing 90% of Ig, and then decline… in mature milk and [represent] 80% of Ig” [10].
IgA is the primary acting antibody on mucosal surfaces. Levels can increase in colostrum in response to a respiratory or gastrointestinal infection a mother had while pregnant, specifically targeting mucosal surfaces and protecting the digestive and respiratory tracts by coating the gut lining and airways in response to pathogens mom has encountered in the environment by binding to and neutralizing these pathogens [13], [14].
How mom’s interactions with her baby help build his or her immune system
IgA also specifically targets any pathogens the baby is currently fighting, developing through a fascinating immunological interplay between mom and baby [15]: the pathogens a baby encounters can actually enter the mother through her contact with the infant’s saliva, feces, skin secretions, and through the nipple during feeding [16], [17], [3]. So that burning desire to kiss your baby all over her sweet little face (and belly, and hands, and toes, and…) can actually influence your breastmilk to build up your baby’s immune system!
IgA also specifically targets any pathogens the baby is currently fighting, developing through a fascinating immunological interplay between mom and baby: the pathogens a baby encounters can actually enter the mother through her contact with the infant’s saliva, feces, skin secretions, and through the nipple during feeding. So that burning desire to kiss your baby all over her sweet little face (and belly, and hands, and toes, and…) can actually influence your breastmilk to build up your baby’s immune system!
But instead of the mother getting ill from these pathogens (since the exposure level is generally quite low and she has a more advanced, adapted immune system) her lymphatic system encounters these pathogens and her aforementioned B cells are triggered to produce IgA. These cells then migrate through her peripheral lymph nodes, spleen, tonsils, and exocrine glands, including her mammary glands, and express IgA into her breast milk to her infant. These antibodies then attach to the infant’s intestinal lining and act to block pathogens before they can take hold and cause infection.
What about formula-fed babies?
While breast milk provides many benefits that formula does not, full-term formula-fed infants do indeed produce their own IgA beginning at 2–4 weeks, but they have to rely more on IgM and prenatal IgG to prevent infections, as these antibodies and their developing immune systems are their main systemic and digestive defenses [18]. Some studies have found that IgM and IgG can even fill in in some cases where IgA is lacking to protect mucosal surfaces [19]. Our bodies are amazing, adaptable things!
The bottom line on building your baby’s immune system
It turns out that even before your baby is born, your body is hard at work to build up her immune system—and for breastfeeding moms, that work continues after birth. It’s another good reminder that postpartum moms are doing more than ever to promote the health and development of their babies, and that they not only need, but truly deserve, the support of their families and communities during the first years of their babies’ lives.
