Does antenatal anti-D help the current baby?

penbabyA number of people have asked me variations on the same question. Does antenatal anti-D help the current baby?

My understanding is that it does not. Anti-D is ‘all about the next baby’.

In this blog post, I explain why that is.

 

Meet the antigens

First, it’s important to know what an antigen is. An antigen is a substance that is foreign to the body. When an antigen is detected by your body, your body mounts an immune response to it. In simple terms, your cell plasma cells start to generate antibodies to counteract the antigen. This is called a primary immune response. It leads to the production of antibodies, which we call immunoglobulin M, or IgM antibodies.

Once these IgM antibodies have counteracted the offending antigen, the body stops producing them. Cleverly, though, your body retains a memory of how it needs to respond if it encounters the same antigen again. That’s only sensible and it’s a mechanism which stops us from passing the same cold around forever. Once somebody has developed antibodies to a particular antigen, if the same antigen does enter the body again, the body mounts what is known as a secondary immune response. This occurs much faster than a primary immune response. Even more importantly for our purposes, the antibodies that it produces will come in a slightly different form. This time, they are called immunoglobulin G or IgG.

This primary response that I described above is exactly what can happen if a rhesus negative women encounters rhesus positive blood.

 

What would happen naturally?

Let’s pretend for a moment that we don’t have any injectable anti-D (rhogam). What would happen ‘naturally’? Well, her body will recognise the anti-D as foreign. When that happens, it will mount a primary immune response. She would make IgM antibodies to the rhesus positive cells. Once the rhesus positive blood cells have all been cleared from her bloodstream, her antibody production will slow down and then stop. But her body will retain the memory of how to create them.

And now we now have a problem.

Because if she becomes pregnant with another rhesus positive baby in the future, her body now has a ‘template’ to make IgG antibodies against the baby’s blood. This can cause problems for a later rhesus positive baby. (It won’t affect a baby who is rhesus negative).

All of this explains why women are offered an injection of anti-D in various situations. The manufactured version (actually a blood product) ‘mops up’ any antigens that she may encounter if her rhesus positive baby’s blood enters her bloodstream. The manufactured anti-D prevents the need for the woman’s body to make natural anti-D antibodies. And that means that future babies are not at risk from those antibodies.

 

Why antibody size matters

A hugely important thing to know at this point is this: IgM antibodies are about six times as big as IgG ones.

IgG antibodies can cross the placenta. IgM antibodies can’t.

So a woman who is mounting a primary immune response to rhesus positive blood cells is not going to be producing the kind of antibody that can cross the placenta to harm her growing baby.

But a woman who is mounting a secondary immune response will produce the far smaller IgG antibodies. Those can cross the placenta.

During the primary immune response which involves the big IgM antibodies, the baby in utero is not at risk.

 

Why timing is key

But the difference between the primary and secondary immune responses is only one part of the answer to this question.

Another element of this is the timeframe of the process of antibody production.Which varies, depending on the antibodies.

Some antibodies can be produced within days. But the primary immune response to the rhesus factor can take up to six months. The rhesus factor isn’t produced until near or during the second trimester of pregnancy. And it can take up to six months for the primary immune response to stop. (Don’t forget that the primary immune response doesn’t pose a threat to the current baby because IgM is too big to cross the placenta).

By the time all of these timeframes have been added together, there’s not much chance of that same baby encountering IgG through a secondary immune response. He or she has almost certainly been born before that happens. Future siblings may encounter problems if they are rhesus positive. That’s because now their mother has a template for creating IgG antibodies that are small enough to cross the placenta into the baby’s bloodstream. But Does antenatal anti-D help the current baby? No. Antenatal anti-D is not about protecting the current baby.

 

Into the unknown

There is still a lot we don’t know in this area. We know that pregnant women have altered immune systems. This is so that their bodies do not reject their babies, as at least half of a baby’s genetic material comes from someone else. (Sometimes more, when assisted fertility methods are used). But we don’t know enough about how that works. We are beginning to form some theories about how the altered immunity of a pregnant woman might be related to some of the problems that pregnant women occasionally face. But there is much we don’t know about maternal and fetal immunity in general. And there are also gaps in our knowledge about anti-D.

 

What don’t we know?

  • As I said above, we know that IgG can cross the placenta. This is generally a good thing. It means that babies in utero gain immunity to things that their mother has already gained immunity to. But it’s not ideal in relation to the creation of anti-D. Given that nature generally gets it right most of the time, is there more that we don’t yet understand about this? Are there things that might affect a woman’s immune response to anti-D?
  • When a woman decides to have manufactured anti-D during pregnancy in order to possibly protect her future babies, this will enter the bloodstream of her current baby. We think that about ten per cent of it crosses the placenta. But we don’t know what effect/s this might have on that baby. It’s possible that it might have a different effect on rhesus positive and rhesus negative babies. We don’t know.

There are always more questions. For now, I hope that this has explained why manufactured anti-D given routinely in pregnancy does not benefit the current baby. It may indeed benefit future ones and it is only one tiny element of a bigger picture.

Please also let me finish with a caveat: I am not an immunologist, and our understanding of this area is developing quickly. If anyone working in this field knows of anything new which may affect this, please let me know.

For lots more information on anti-D and related decision-making, see my book 🙂Anti-D in Midwifery: Panacea or Paradox?

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