How Long Do Babies Have Their Mother's Immune System?

There are two categories of immunity: passive and active immunity. These labels refer to how your body develops protection against a certain disease or toxin. 

Passive immunity. This type of immunity occurs when a person is given antibodies -- the body's response to a pathogen -- rather than producing them through their own immune system. Newborn babies get passive immunity from their mothers through the placenta before they're born. A mother shares the antibodies that she has with her unborn baby.

Each baby's passive immunity is unique because each mother's immune system is unique. The exact kinds and amounts of antibodies in a baby's blood are a reflection of its mother's levels at birth. These differences can impact how long passive immunity lasts. 

Active immunity. This type of immunity occurs when your body is exposed to a particular disease and develops antibodies to fend it off. Active immunity can happen in two ways: naturally or vaccine-induced. 

When you're exposed to a pathogen (like a bacteria or virus) in the world, your immune system learns to "recognize" that pathogen and can protect you in the future. If you receive a vaccine, that's a way to "teach" your immune system safely how to do the same thing -- learn to recognize the pathogen and protect yourself from it if you're ever exposed. 

Babies begin to build active immunity shortly after birth as they are exposed to germs in the outside world and as they begin to receive the necessary immunizations to keep them healthy.

Studies are still ongoing in this subject, so there's no precise answer. The exact amount of protection that a baby receives from its mother depends on the antibodies that the mother has in her immune system. Research indicates that a baby's passive immunity lasts for around six months.

One study examined the passive immunity to measles in infants. It discovered that the babies' immunity to the disease diminished over time, and none of them had immunity by the age of 9 months. 

In other words, it isn't like flipping a switch. There isn't a cutoff date at which a baby is no longer protected by the passive immunity provided by its mother. Rather, immunity to certain pathogens slowly decreases over several months after birth. 

Breast milk, however, contains antibodies that can extend a baby's ability to fight infection. 

Among other important elements, breast milk has the antibodies that a mother has in her body (from past infections) as well as those she makes in response to illnesses that she encounters while breastfeeding. This extra support for the developing immune system can help keep newborn babies healthy while their passive immunity wanes and their own immune systems continue to grow stronger. 

Studies also show that antibodies aren't the only leg up babies get when developing the components of their own immune systems. One study demonstrated that breastfeeding actively encourages an infant's ability to develop critical elements like lymphocytes, cytokines, and other key pieces of a strong immune system.  

Components of breast milk are also directly anti-inflammatory, which helps a newborn's ability to manage their immune response to specific germs, as well as protecting against autoimmune conditions like celiac disease and certain kinds of allergies.

Studies have shown that babies who are breastfed -- especially those breastfed by vaccinated mothers -- experience fewer infections both in the developing world as well as developed nations like the United States.