Babies’ Immune Systems: What Can They Handle?

Dr. Offit explains whether babies’ immune systems can handle the challenges they encounter in the first few months after birth and how vaccines are designed to help them overcome some particular challenges.


Paul Offit, MD: Hi, my name is Paul Offit. I am talking to you today from the Vaccine Education Center at the Children’s Hospital of Philadelphia. And I thought we would talk about babies, specifically whether babies can handle the challenges that they have to handle in the first few months of life after they come out of the womb. Can they really handle the onslaught of bacteria that they invariably encounter? So, we’ll start from the beginning. When babies are in the womb, they’re in a sterile environment, so they don’t have any bacteria that live on the surface of their skin or in their nose or in their throat or their intestines. And, assuming that there hasn’t been a problem with the pregnancy, they have never been exposed to viruses. So, they have never been exposed to viruses, and they’ve never been exposed to bacteria. When they’re born, however, when they enter the vaginal canal and then the world, they very quickly come in contact with trillions of bacteria, which then start to live on the lining of their nose, their skin, their throat, and their intestines. In addition, they start to come in contact with viruses.

Can they handle all that?

For the most part, they can. But there are two ways in which babies are immunologically deficient. The first is their ability to make pus. So, pus is made by white cells. The specific kind of white cells that comprises pus is called a neutrophil. And the capacity of babies’ neutrophils to go to areas of infection and then kill bacteria is less than an older child, or even an older infant. And that’s why when babies have fever in their first few weeks of life, it’s very important to bring them to the doctor to make sure that they’re not being infected by one of these bacteria. That’s something by … really by 2 months of age, children now can make neutrophil, or pus responses, similar to older infants. So, that’s neutrophils.

The second thing where babies are immunologically less capable than an older child is in how they make an antibody response. So, antibodies are those proteins that your body makes to attach to bacteria or to attach to viruses to try and neutralize them. The cell that helps these B cells make antibodies … so B cells make antibodies then there’s a type of cell called T cells which help B cells make antibodies. So B cells make antibodies, T cells help B cells make antibodies. There are some types of immune responses to bacteria that have these complex sugars on the surface called polysaccharides, where it’s very difficult for a baby to make that immune response. So, when you’re trying to make an immune response to these complex sugars on the surface of bacteria, that’s a T cell-independent response, and babies aren’t very good at that. So, bacteria like pneumococcus or Haemophilus influenzae type b or meningococcus, those babies don’t make good immune responses to those bacteria because they are coated with this complex sugar called a polysaccharide.

So if that’s true then, why is it that we have vaccines to pneumococcus? Why is it that we have a vaccine to Haemophilus influenzae type b? And the reason that we can do that is that we take the complex sugar from pneumococcus or from Haemophilus influenzae type b and we link it to a harmless protein. What that does is it converts a T cell-independent B cell response to a T cell-dependent B cell response. So babies can do that. They can make T cell-dependent B cell response; they just can’t make T cell-independent B cell responses. So for example, the hepatitis B vaccine is just proteins from hepatitis B to which a baby can make a response because that’s a T cell-dependent response, and they can do that. The way that they make a response to these bacterial vaccines we give them is by linking it to a protein; we also make that a T cell-dependent response, so we get around the problem that the baby has making a T cell-independent response.

So in summary, babies have two problems in terms of immune deficits. One is that they’re not very good at making pus early on in their first few weeks of life, so they are at risk of bacterial infections. And they aren’t very good at making immune responses to bacteria that have these polysaccharides, or complex sugar coatings. The way we get around that in terms of making vaccines is we actually convert that inability to make a so-called B cell-only response, the so-called T cell-independent response, to a T cell-dependent response by linking it to that harmless protein, and that enables us to do something the baby can’t do. The baby can’t typically make a good immune response to those complex sugar-coated bacteria. But by constructing vaccines the way we can, we give them immunity that would be very hard for them to acquire naturally.

Thank you.

Related Centers and Programs: Vaccine Education Center