IgG is the only antibody class that significantly crosses the human placenta, and it is the main source of immune protection a baby receives before birth. After delivery, breast milk supplies a different antibody, secretory IgA, which coats the infant’s gut and respiratory tract. Together, these two transfer routes give newborns a borrowed immune defense that lasts for the first several months of life, bridging the gap before a baby’s own immune system is mature enough to respond to infections and vaccines on its own.
IgG and the Placenta
Of the five classes of antibody the human body makes, only IgG can cross the multiple cell layers of the placenta in meaningful amounts.1PubMed Central. IgG placental transfer in healthy and pathological pregnancies That selectivity is not accidental. A specialized receptor called the neonatal Fc receptor (FcRn) sits on the surface of the placenta’s outermost cell layer. This receptor grabs IgG molecules from the mother’s blood, ferries them through the cell in small internal compartments, and releases them on the fetal side.2The Journal of Immunology. Isolation from human placenta of the IgG transporter, FcRn, and localization to the syncytiotrophoblast: implications for maternal-fetal antibody transport The receptor works by exploiting a pH trick: it binds IgG tightly in the mildly acidic interior of the cell’s transport compartments but lets go once it reaches the neutral pH of fetal blood. Other antibody types, like IgM and IgA, do not bind FcRn and so cannot hitch the same ride.
The result is remarkably efficient. By the time a full-term baby is born, the concentration of IgG in the baby’s blood typically matches or slightly exceeds the mother’s level. Every IgG the mother carries, whether it was made in response to a childhood infection, a recent vaccine, or a chronic exposure, has the potential to be copied into the fetal circulation. That means the baby is born with a snapshot of the mother’s own immune history.
When Transfer Ramps Up
Placental IgG transfer is not constant across pregnancy. It begins modestly in the second trimester and then accelerates sharply, with the highest levels of transfer occurring during the final weeks before birth.3PubMed Central. Selective transfer of maternal antibodies in preterm and fullterm children This timing has a direct consequence for babies born early. A baby delivered at 28 weeks has had far less time in the peak transfer window and enters the world with substantially lower antibody levels than a baby born at 40 weeks. Preterm infants are therefore more vulnerable to infections in their first months, not only because their immune systems are less mature but because they started with a smaller stockpile of borrowed antibodies.
This third-trimester acceleration also explains why the timing of maternal vaccination matters so much. A vaccine given too close to delivery may not leave enough time for the mother to build a robust antibody response and for those antibodies to cross the placenta in sufficient quantity.
Not All IgG Crosses Equally
Even within the IgG class, the placenta plays favorites. IgG comes in four subtypes, numbered IgG1 through IgG4, and they do not all bind the FcRn receptor with the same affinity. IgG1, which makes up the largest share of circulating IgG and carries most vaccine-induced and antiviral antibodies, transfers the most efficiently. IgG2, which tends to target bacterial sugars, crosses less well. Research on maternal RSV vaccination, for instance, found that the antibodies reaching the fetus were heavily skewed toward IgG1.4PubMed Central. Immune Mechanisms Underlying Neonatal Protection Following Maternal RSV Vaccination
Beyond subtype, the sugar molecules attached to each antibody’s tail region also influence transfer. Research has shown that antibodies carrying particular sugar structures, specifically those with two galactose molecules attached, are preferentially shuttled across the placenta.5Cell. Selective Transfer of Natural Killer Cell-Activating Maternal Antibodies Encodes a Placental Sieving Mechanism These particular antibodies happen to be especially good at activating natural killer cells, a type of immune cell that can destroy infected cells on contact. The implication is that the placenta is not just a passive filter. It acts more like a sorting system that favors antibodies with the most potent effector functions, arming the fetus preferentially with the tools most useful for fighting infections right away.6Cell. Fc Characteristics Mediate Selective Placental Transfer of IgG in HIV-Infected Women
Secretory IgA in Breast Milk
Once a baby is born, the placental supply line shuts down. Breast milk takes over as the next source of maternal antibodies, but the star player shifts from IgG to secretory IgA (SIgA). Unlike IgG, SIgA is not designed to circulate in the blood. It is built to survive the harsh environment of the gut and coat mucosal surfaces, acting as a kind of antiseptic paint on the lining of the intestines and, to a lesser extent, the airways.
SIgA in breast milk reflects the infections and microbial exposures the mother has encountered, especially through her own gut and respiratory tract. When a mother is exposed to a pathogen, immune cells in her intestinal lining travel to the mammary gland and produce SIgA specific to that pathogen, which then ends up in milk. This pathway means breast milk antibodies are particularly well-tailored to the germs circulating in the mother’s and baby’s shared environment.
Animal studies have shown that SIgA in breast milk does more than just block pathogens. It shapes the composition of the baby’s developing gut microbiome. Mice that received maternal SIgA through nursing had significantly different gut bacteria compared to those that did not, and those differences persisted into adulthood.7PubMed Central. Secretory antibodies in breast milk promote long-term intestinal homeostasis by regulating the gut microbiota and host gene expression SIgA also prevented harmful bacteria from escaping the gut and spreading to lymph nodes in newborn animals. The picture that emerges is that breast milk antibodies play a dual role: defending against infection and helping curate the bacterial communities that will be the infant’s permanent residents.
Breast milk also contains small amounts of IgG and IgM, though in much lower concentrations than SIgA.8PubMed Central. Adiposity-dependent adipokine-immunoglobulin interplay in mature breast milk The functional importance of these trace antibodies in milk is still being studied, but their concentrations are far too low to substitute for the IgG that a baby would normally receive across the placenta.
Beyond Antibodies in Breast Milk
Breast milk is not just an antibody delivery vehicle. It also contains live immune cells, including a substantial number of cytotoxic T lymphocytes, the same cells your body uses to kill virus-infected cells directly. Research in animal models found that these maternal T cells survive passage through the infant’s stomach, reach specialized immune tissue in the gut called Peyer’s patches, and are functionally more potent than the infant’s own immature T cells.9PubMed Central. Transfer of Maternal Immune Cells by Breastfeeding: Maternal Cytotoxic T Lymphocytes Present in Breast Milk Localize in the Peyer’s Patches of the Nursed Infant These transferred cells appear to home specifically to gut immune tissue, suggesting they are tailor-made to patrol the site where the infant faces the greatest infection risk.
Breast milk also carries stem cells, cytokines, and other signaling molecules that influence how the baby’s immune system develops. Transfer of maternal cells during lactation has been linked to the expansion of regulatory T cells in the infant, tolerance to the mother’s tissue markers, and even stronger responses to vaccines.10World Journal of Peri & Neonatology. Breast Milk Stem Cells and Maternal Microchimerism: Mechanisms and Clinical Implications The full scope of what maternal cells accomplish in the nursed infant is still being mapped, but it is clear that breast milk offers a much broader immune package than antibodies alone.
Maternal Vaccination and Antibody Transfer
Because the placenta will transport whatever IgG the mother has circulating, vaccination during pregnancy is a deliberate strategy to load the baby with specific protective antibodies before birth. Influenza, pertussis (whooping cough), RSV, and COVID-19 vaccines given during pregnancy all generate antibodies that cross the placenta and are detectable in cord blood at delivery.
Timing matters. For influenza, vaccination in either the second or third trimester produced significantly higher antibody levels in cord blood compared to babies of unvaccinated mothers.11PubMed Central. The impact of timing of maternal influenza immunization on infant antibody levels at birth For RSV, the interval between vaccination and delivery turned out to be critical: when the RSV vaccine was given only two to three weeks before delivery, the ratio of cord blood antibody to maternal antibody was significantly lower than when vaccination occurred more than five weeks before delivery.12PubMed Central. Enhanced placental antibody transfer efficiency with longer interval between maternal RSV vaccination and birth The body needs time to mount a full antibody response, and then the placenta needs time to actively transport those antibodies. Vaccinate too late, and the baby misses the benefit.
The general recommendation for pertussis vaccination during pregnancy, for instance, targets the early third trimester specifically to optimize the time available for both antibody production and placental transfer.13PubMed Central. Experience and challenges on influenza and pertussis vaccination in pregnant women
How Long Maternal Antibodies Last
Maternal IgG does not persist in the baby indefinitely. The baby’s body does not replenish it; it simply degrades over time. A large meta-analysis pooling individual-level data from multiple studies estimated the half-life of maternal antibodies against diphtheria, tetanus, and pertussis at roughly 29 to 35 days, depending on the specific antigen.14PubMed Central. The half-life of maternal transplacental antibodies against diphtheria, tetanus, and pertussis in infants: an individual participant data meta-analysis That means roughly every month, the antibody concentration in the baby’s blood drops by half. By about three to six months, levels have usually fallen below the threshold needed for reliable protection, which is why infant vaccination schedules typically begin around two months of age.
There is an interesting wrinkle for preterm infants. One study found that the half-life of pertussis-specific antibodies was somewhat longer in preterm babies than in those born at full term, on the order of 32 days versus 24 days for one pertussis antigen.15The Journal of Infectious Diseases. Half-life Estimation of Pertussis-Specific Maternal Antibodies in (Pre)Term Infants After In-Pregnancy Tetanus, Diphtheria, Acellular Pertussis Vaccination That slower decay may partially offset the lower starting levels that preterm infants receive, but it does not fully close the gap. Preterm babies still end up with less total protection during the vulnerable early months.
The Blunting Effect on Infant Vaccines
Maternal antibodies are not a free lunch. High levels of maternal IgG circulating in a baby at the time of vaccination can actually blunt the infant’s own immune response to the vaccine. The mechanism is straightforward: the mother’s antibodies bind the vaccine antigen before the infant’s immune system gets a chance to respond to it, effectively hiding it from the baby’s B cells. This phenomenon has been documented across many vaccine types, including both standard protein-based vaccines and newer approaches like live viral vectors.16Vaccine. Determinants of infant responses to vaccines in presence of maternal antibodies
A study tracking infants whose mothers received the pertussis-containing vaccine during pregnancy found that those babies had reduced antibody responses not only to pertussis and diphtheria but also to apparently unrelated vaccine antigens like polio and several pneumococcal types. The blunting effect was more pronounced at 13 months of age than at 7 months.17The Lancet. Influence of maternal immunisation on infant vaccine responses: an observational study Animal models of rotavirus vaccination have confirmed the same basic dynamic: pups vaccinated in the presence of maternal antibodies failed to mount their own detectable antibody response.18PubMed Central. Mechanisms of maternal antibody interference with rotavirus vaccination
This creates a balancing act for public health. Vaccinating pregnant women protects newborns during their most vulnerable first weeks, but it may slightly reduce the effectiveness of the baby’s own vaccine series months later. In practice, the consensus is that the early protection outweighs the blunting risk, and infant vaccine schedules with booster doses are designed to overcome it. But it is a real trade-off, and it is one reason why researchers continue to fine-tune the timing and formulation of both maternal and infant vaccines.
When Maternal Antibodies Cause Harm
The placental transport system does not distinguish between helpful antibodies and harmful ones. If a mother has autoimmune antibodies, those IgG molecules cross the placenta just as efficiently as protective ones. The best-known example is neonatal lupus. Mothers who carry anti-Ro/SSA or anti-La/SSB autoantibodies, often associated with lupus or Sjögren syndrome, can transfer these to the fetus, where they may cause a rash, liver problems, or in the most serious cases, congenital heart block.19PubMed Central. Molecular Mechanisms of Fetal and Neonatal Lupus: A Narrative Review of an Autoimmune Disease Transferal across the Placenta Most manifestations of neonatal lupus resolve on their own as the mother’s antibodies are cleared from the baby’s circulation, but congenital heart block can cause permanent damage.20PubMed. Maternal autoantibodies and pregnancy–II: The neonatal lupus syndrome
A similar issue arises with therapeutic antibodies. Biologic drugs used to treat conditions like inflammatory bowel disease, such as infliximab and adalimumab, are engineered IgG molecules. They cross the placenta by the same FcRn pathway as natural antibodies. In fact, levels of infliximab in cord blood reached about 160% of the mother’s level, and adalimumab about 153%. Both drugs remained detectable in the infant’s blood for as long as six months after birth.21PubMed Central. Placental Transfer of Anti-Tumor Necrosis Factor Agents in Pregnant Patients with Inflammatory Bowel Disease Because these drugs suppress parts of the immune system, infants exposed in utero are typically advised to delay live vaccines until the drug has cleared. One notable exception is certolizumab, an engineered antibody fragment that lacks the Fc region needed to bind FcRn. Its levels in cord blood were negligible, around 4% of the mother’s concentration.
What Impairs Transfer
Several maternal infections can reduce the efficiency of placental antibody transfer. HIV, malaria, cytomegalovirus, and SARS-CoV-2 have all been linked to lower-than-expected antibody levels in cord blood.22PubMed Central. Impact of Infections During Pregnancy on Transplacental Antibody Transfer The mechanisms vary. In HIV, the mother’s total IgG levels are often elevated, which can saturate the FcRn receptor and reduce the proportion of any single specific antibody that gets through. Malaria can damage the placenta directly. In SARS-CoV-2, altered antibody glycosylation patterns may reduce binding efficiency.
Systemic inflammation, even without a specific pathogen, can also interfere. A recent study found that higher levels of the inflammatory markers TNF-alpha and IL-6 in the mother’s blood were associated with reduced antibody transfer, regardless of whether the mother had HIV.23PubMed Central. Maternal inflammation disrupts transplacental antibody transfer in pregnant individuals with and without HIV Interestingly, inflammation visible under the microscope in the placental tissue itself did not predict transfer efficiency in that study. It was the circulating inflammatory signals, not the local tissue damage, that seemed to matter most.
Donor Milk and Pasteurization
For premature or sick infants who cannot receive their own mother’s milk, donor milk from a milk bank is often the next best option. But donor milk is pasteurized before distribution, and that heat treatment takes a toll on its immune components. Research has confirmed that pasteurization significantly alters the immunological composition of breast milk, with some components reduced drastically.24PubMed. Impact of Holder pasteurization on immunological properties of human breast milk over the first year of lactation SIgA levels are partially preserved, but many of the live immune cells, cytokines, and other bioactive molecules are destroyed or reduced. Donor milk still provides nutritional benefits and some passive immunity, but it is immunologically weaker than fresh mother’s own milk. This is one reason neonatal care teams prioritize a mother’s own expressed milk over donor supplies when possible.
How Humans Compare to Other Mammals
The human reliance on placental IgG transfer is not universal among mammals. Cows, pigs, and horses have a placental structure that does not allow antibody passage at all. Their newborns arrive with essentially zero circulating maternal antibodies and depend entirely on colostrum, the thick first milk produced after birth, to receive a massive dose of IgG through the gut during the first hours of life. A calf that misses its colostrum window faces severe, often fatal, immune vulnerability. Rodents, meanwhile, use a mix of both routes, acquiring antibodies before and after birth.25PubMed. Advancing protective effects of maternal antibodies in neonates through animal models
In humans, colostrum is rich in SIgA rather than IgG, and the newborn’s gut loses the ability to absorb intact antibodies into the bloodstream within the first day or two. So for humans, colostrum serves mainly as a mucosal coating for the gut rather than a systemic IgG delivery mechanism. The evolutionary split is interesting: species whose placentas block antibody transfer evolved a gut that can absorb it, and vice versa. Humans bet heavily on the placental route, which is why preterm birth carries such an outsized immunological penalty.