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The Maternal Immune System
Claire Mazzia, BSc & Kelsey McLaughlin, PhD

The immune system plays an important part in helping the mother’s body adapt to pregnancy, contributing to a healthy pregnancy outcome for both mom and baby. In addition to keeping mom safe from infection during pregnancy, the pregnant woman’s immune system supports growth of the placenta and also prevents the rejection of the baby as a foreign object through local immune suppression in the uterus.

The 5 most common questions relating to the immune system and pregnancy are:

  1. What is the immune system?

  2. How does the immune system change during pregnancy?

  3. Are women more likely to get sick while pregnant?

  4. Which vaccines should pregnant women get?

  5. What is the RhoGAM vaccine?

Keep reading to learn more!

1. What is the immune system?


The immune system is made up of several organs and types of cells that work together to fight infection and disease in order to keep the body healthy. We encounter numerous types of bacteria, viruses and other germs every day, as well as other types of foreign invaders such as parasites. If the immune system is healthy and working properly to protect our body, we can typically fight off invaders.


Interested in learning more about the organs and cells that make up the immune system? There is more detailed information at the bottom of this article. 

2. How does the immune system change during pregnancy?

The immune system is highly involved in supporting a normal pregnancy.


In non-pregnant women, the immune system view outside invaders as a threat that could cause infection. The growing fetus could technically be viewed as a biological invader, since the fetus contains genetic material from the father that is different than the mother's genetic material. Amazingly, the mother’s immune system adapts in normal pregnancy to prevent an attack against the fetus, supporting a healthy pregnancy. 


The immune system also helps to support the development of the placenta. Early in pregnancy, immune cells promote placental development in the uterus. The placenta acts as a barrier between the mother and the fetus, providing nourishment and protection to the baby. The mother’s blood is not directly in contact with the fetus, however, the mother’s blood is in direct contact with the placenta. Interestingly, the mother's immune system does not recognize the cells of the placenta as being foreign, even though these placental cells are more similar genetically to the baby than the mother. This lack of recognition by the mother’s immune system prevents an immune attack on the unborn child. 

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3. Are women more likely to get sick while pregnant?


Pregnancy is a unique immune condition. There is some uncertainty whether the mother's immune system is slightly suppressed during pregnancy, which would make women more vulnerable to certain infections. This theory is supported by the observation that women who suffer from autoimmune diseases (activation of the immune system) typically find that pregnancy improves their autoimmune symptoms, potentially meaning that their immune system is suppressed. Increased levels of certain hormones during pregnancy, such as estrogen and progesterone, may cause changes to the mother’s immune system and response to potential invaders.


The immune system of pregnant women is a balance of localized suppression in the uterus to allow for the healthy growth and development of the baby, while still protecting the mother from harmful infections. Due to the unique immune status of pregnant women, their immune system deals with potential infections differently than those who are not pregnant, potentially making them more likely to develop certain infections, such as the flu and malaria

Infections in pregnant women usually present more severely than in non-pregnant women, and there is an increased risk of complications as a result of the infection. Exposure to viruses while pregnant can stimulate a mild inflammatory response by the placenta, which can then activate the mother’s or unborn baby’s immune system, impacting maternal and/or fetal health.

There are some illnesses which pregnant women are particularly vulnerable to, including:

  • Listeriosis: Listeria is a contaminant most commonly found in uncooked foods including raw meats, vegetables or unpasteurised milk or soft cheeses. Listeriosis is much more common in pregnant women, than the general population. Infection is associated with severe impacts on the baby's health, including increased risk of pneumonia, sepsis, meningitis and fatality. 

  • Toxoplasmosis: The Toxoplasma gondii parasite is commonly found in the digestive system of animals like birds and cats, and usually does not harm humans unless their immune system is significantly weakened. If pregnant women acquire this infection, it can be transmitted across the placenta to the fetus, whose immune system is not yet developed. It is recommended that pregnant women avoid contact with litter boxes and soil, which may contain cat feces.

4. Which vaccines should pregnant women get?


The purpose of vaccines during pregnancy is to strengthen the immune system against certain infections to protect the mom and fetus. Vaccines contain weakened or inactive viruses that stimulate your body to produce antibodies for protection against that virus, without becoming infected by the virus. These antibodies remain in your blood so that if you are ever exposed to the invader in the future, they will quickly recognize and kill the pathogen so you don’t become ill. 

Vaccinations during pregnancy also provide babies with protection during the first few months of life

The Society of Obstetricians and Gynaecologists of Canada makes the following recommendations: 

  • Tetanus, diphtheria, and pertussis vaccine (Tdap) for all pregnant women

  • Hepatitis B, hepatitis A, meningococcal, and/or pneumococcal vaccines for pregnant women who have specific risk factors 

  • Live and/or live-attenuated vaccines are not recommended for pregnant women; inactivated vaccines, bacterial vaccines, and toxoids can be used safely in pregnancy


5. What is the RhoGAM vaccine?


Humans have 45 types of blood groups. When a blood type is reported (ie. O positive), the 'positive' or 'negative' refers to their Rh(D) status, with Rh being one of the 45 blood groups.

If a pregnant woman is Rh(D) negative and the baby is Rh(D) positive, this can cause the mother's body to have an immune response against the fetus, specifically the baby's red blood cells.


To prevent this immune attack, women who are Rh(D) negative are recommended by The Society of Obstetricians and Gynaecologists of Canada to receive a vaccine which contains proteins that bind to the Rh molecules of the baby’s blood, preventing the mother’s body from reacting to them. Pregnant women who are Rh(D) negative are recommended to receive the RhoGAM vaccine at 28 weeks of pregnancy, as well as within 72 hours following labour if the baby tests Rh(D) positive.

A mother should not receive this vaccine if she is Rh(D) positive.

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Interested in learning more about the maternal immune system? Keep reading!

Which organs & cells make up the immune system?

The adaptive and innate immune systems involve organs including bone marrow, the thymus, lymph nodes, spleen, gut, and tonsils.


White blood cells, a key component of the immune system, develop from stem cells in the bone marrow. There are five general types of white blood cells that have a variety of functions. Neutrophils, the most abundant type of white blood cell are phagocytes (‘phago’ meaning ‘eating’ and ‘cytes’ meaning ‘cells’). These cells engulf potentially harmful foreign invaders and break them down to neutralize the threat of infection.


Lymphocytes such as B-cells and T-cells are part of your adaptive immune system. These cells develop throughout your life as you encounter more germs. Lymphocytes can develop into memory cells to prevent reinfection by the same pathogen by recognizing a specific invader and efficiently killing it. Eosinophils are normally present in low levels in the blood and increased levels are associated with parasitic infections. Monocytes can differentiate into several different cell types that are involved in identifying foreign invaders and organizing a systemic immune response by communicating with other cells such as lymphocytes. Basophils are activated during an allergic response.


All these cells are found in the blood and different tissues, and are constantly surveying the body for pathogens or unhealthy cells to help prevent you from getting sick, and promote recovery from an illness like a cold or the flu. These cells work together by sending signals to each other to help organize an effective immune response against foreign organisms.


  • Bone marrow is a spongy tissue inside bones where white blood cells grow and develop.


  • The thymus, situated just above the heart, is where a specific type of white blood cells called T-cells develop.


  • Lymphatic vessels are a network that span the entire body, like the blood stream, but they carry white blood cells to immune organs. They are also connected to the circulatory system.


  • Lymph nodes are very small bean-shaped organs that connect lymphatic vessels. They are found all around the body and are a meeting point for white blood cells to communicate and activate other cells to attack the foreign invader. 

How does the immune system protect the body from infection?

The immune system consists of two parts that work together to protect the body:


     1. The innate immune system


We are all born with an innate immune system, which is made up of different types of chemical and physical barriers to protect the body against disease. Examples of these barriers include parts of the body that interact with the outside world (e.g. skin, stomach) and parts of the body that create an environment that prevents germs from causing an infection (e.g. the vaginal environment is an acidic barrier against infection).


The innate immune system consists of special white blood cells called phagocytes. These cells engulf potentially harmful foreign invaders and break them down to neutralize the threat of infection. If the innate immune system needs to recruit more white blood cells to a particular area in the body to fight off infection, it will signal the adaptive immune system for more support.


     2. The adaptive immune system


The adaptive immune system develops throughout your life, as you are exposed to more germs. After the adaptive immune system has interacted with an invader, it is able to remember that invader in the future, and can successfully attack it to prevent recurring infection. For example, you can only get the chicken pox once because your cells recognize and attack the infection if you ever come into contact with it again. More specifically, memory immune cells have specific receptors that match the proteins on pathogens the body has previously encountered, meaning the body sends off an alarm if it ever comes in contact with it again to mount a highly organized and efficient immune response.


However, some germs like viruses that can cause colds are able to mutate quickly, meaning that the body is not able to recognize them as a previous invader and cannot kill them as quickly. As a result, you are more likely to get re-infected by the same invader.


Antibodies are an essential component of the adaptive immune system: they are proteins found in the blood that bind to harmful pathogens, effectively neutralizing them and helping the body recover. During an infection immune cells called B-cells secrete many antibodies into the blood that attack the organism causing the infection. They also stimulate the ‘eating cells’ called phagocytes to engulf and breakdown pathogens. They are extremely effective and efficient at fighting disease because they are produced in large amounts and have a region that has a high affinity to the foreign organism so they can quickly recognize and neutralize the destructive invader. 

How do vaccines work?


The purpose of vaccines is to prevent people from getting sick from a specific disease before they come into contact with it. Vaccines contain weakened or dead invaders that would normally cause you to become sick, but instead, stimulate antibody production from B-cells. The antibodies remain in your blood so that if you are ever exposed to the invader in the future, they will quickly recognize and kill the pathogen so you don’t become ill.


There are four different types of vaccines:

  • Live-attenuated vaccines

  • Inactivated vaccines

  • Toxoid vaccines

  • Subunit, recombinant, polysaccharide, and conjugate vaccines


Live attenuated vaccines contain the live weakened pathogen and result in long-lasting immunity after one or two doses because it closely resembles the natural germ. An example of live-attenuated vaccine is measles mumps and rubella (MMR). Inactivated vaccines such as the flu shot contain the dead invader, and boosters are required to ensure immunity is maintained throughout one’s lifetime. Toxoid vaccines are designed to promote immunity against the parts of the pathogen that induce disease. For example, the tetanus bacteria produce a dangerous toxin that damages the nervous system and the vaccine induces immunity against the toxin. Subunit, recombinant, polysaccharide, and conjugate vaccines such as hepatitis B or HPV contain a specific portion of the virus such as a sugar or protein that stimulates a strong immune response. Booster shots may be required, and patients with immune deficiencies or illnesses can usually receive these vaccines as they do not contain the whole virus.  


Every woman and every pregnancy is unique. Pregnant women should speak to their healthcare provider to ensure maternal and fetal safety. This article is meant to provide readers with current information and opinions. All medical and treatment decisions should be discussed with your healthcare provider.

This article was written by Claire Mazzia and Dr. Kelsey McLaughlin and edited by Dr. Kelsey McLaughlin and Dr. Melanie Audette.

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