The immune system undergoes a profound and highly coordinated change during pregnancy, often misinterpreted as a simple weakening. This necessary and complex biological shift is termed immune modulation or tolerance. This intricate process is designed to protect the developing fetus, which the maternal immune system would otherwise recognize as partially foreign. The immune system is not universally suppressed but rather reprogrammed, balancing the need to sustain the pregnancy while maintaining a defense against external pathogens.
Immune Modulation: Why the System Changes
The fundamental reason for this immune shift is the genetic makeup of the fetus, which is considered a semi-allograft because it inherits half of its genes from the father. Without a dedicated mechanism to prevent rejection, the mother’s immune system would attack the developing embryo. The body must establish immune tolerance at the maternal-fetal interface, the boundary where the placenta meets the uterine wall. This deliberate, proactive process ensures the survival of the pregnancy, rather than being a passive failure of the immune system.
The success of gestation relies on the immune system actively supporting the pregnancy. Regulatory T cells (Tregs), a specialized immune cell type, are expanded and play a primary role in this tolerance. These cells migrate to the uterine lining (decidua), where they suppress the rejection-focused responses of other maternal immune cells. Hormonal changes, particularly high levels of progesterone and estrogen, also contribute by modulating immune cell function and promoting a less inflammatory state.
Selective Suppression and Activation: The Immune Shift
The mechanism making the pregnant person susceptible to certain illnesses is the shift in T-helper cell dominance. The immune system moves away from a T-helper 1 (Th1) profile toward a T-helper 2 (Th2) profile for much of the pregnancy. Th1 responses are characterized by cellular immunity, deploying pro-inflammatory agents like interferon-gamma to fight intracellular pathogens, such as viruses and some bacteria. The suppression of this Th1 activity creates the perceived vulnerability to specific infections.
Conversely, the Th2 response is a humoral or antibody-mediated immunity that is generally maintained or enhanced. This antibody production is less destructive to the fetal-maternal interface and is necessary for overall immune defense. The Th1/Th2 balance is precisely regulated, allowing the maternal body to tolerate the fetus while still performing immune surveillance. Innate immune cells, like Natural Killer (NK) cells, are not suppressed but change their function, transforming into decidual NK cells that actively promote the growth of placental blood vessels.
Increased Risk: Infections to Monitor
This selective shift in immune function explains why pregnant people face an increased risk or severity from certain diseases. Infections requiring a strong Th1-mediated cellular response for clearance pose a greater threat. Influenza, for instance, can lead to more severe illness, a higher rate of hospitalization, and increased mortality in pregnant individuals compared to non-pregnant adults. This heightened severity is attributed to the dampened cellular response combined with physiological changes in the respiratory system.
Specific pathogens typically contained by a cellular response also become more concerning. Listeriosis, a foodborne illness caused by the bacterium Listeria monocytogenes, is one example where susceptibility is increased due to the immune state. Similarly, a primary infection with Toxoplasmosis, a parasitic infection, is more likely to be transmitted to the fetus due to the altered immune environment. These infections, while rare, highlight the practical health consequences of Th1 suppression.
Maintaining Immune Health While Pregnant
Supporting overall immune function during pregnancy involves focusing on foundational health practices and proactive steps for protection. Adequate, restorative sleep is important, as the body uses this time to produce and consolidate immune memory. A balanced diet rich in micronutrients supports the function of immune cells. Taking a high-quality prenatal vitamin helps ensure requirements are met for both the mother and the developing fetus.
Maintaining strict hygiene, particularly frequent hand washing, is a simple and effective measure to reduce exposure to common respiratory and gastrointestinal pathogens. Food safety is paramount, especially avoiding unpasteurized products and deli meats to mitigate the risk of listeriosis. Staying up-to-date with vaccinations, such as the flu shot and the Tdap vaccine, provides direct, antibody-based protection for the mother and indirect protection for the newborn.