How are babies made? The really cool part

I wrote a post about a month ago about how scientists should communicate with the general public more and are the best people to spark interest in their chosen subject. I think it’s time I put my money where my mouth is and have a go myself! My favourite part of science is reproduction (yes – I have heard most of the jokes) and in particular how the female body’s immune system adapts when she is pregnant.

Have you ever thought about how we reproduce? Two cells meet in a uterus and then they make every single kind of cell in the human body. Two cells make everything. When I think about that I get a rush and I feel like a kid watching fireworks again. What’s amazing to me is that so much happens in such a short space of time. All it takes sometimes is one thing to not be there or go wrong and a baby can’t develop, but look how many times it has worked!

Even if everything is fine with the developing embryo, where it develops is also really important. Having never had children I find the whole idea of having another human being inside you a bit strange but scientifically this is where it gets interesting for me. How does something that is essentially a foreign object survive, much less develop in the womb?

If I transplanted a liver into someone their body would reject it. This is because that liver is recognised as foreign by the recipient’s immune system, which would try to destroy it. When a mammal is pregnant the offspring she’s carrying only shares half her genes, the other half are paternally inherited. Half of an embryo’s genes are foreign to its mother, in exactly the same way as the liver in my example. That means that the mother’s immune system should target the embryo and destroy it. Obviously something stops that from happening but what is that?

When this question was first asked scientists came up with three possibilities, the maternal immune system did not recognise the foetus, there were no markers of foetal cells to be recognised or there was a physical barrier between the two. Over the years all three have been disproven but they weren’t a million miles wrong, the foetus recognisable and exposed to the maternal immune system. Instead it relies on many different tactics to avoid a hostile response.

Firstly hormones released during pregnancy alter the immune system throughout the whole body. Briefly your immune system has two parts. There’s the part that uses immune cells to kill other body cells (for example when that body cell is infected by a virus) and then the part that makes antibodies to fight against bacteria that live outside cells and parasites. Imagine these two scenarios are the opposite end of a scale, both are happening in your body right now but how much of each there is depends on the threat your immune system is currently facing. The first catergory, known as cellular immunity is most dangerous to an embryo, so it pushes the immune response towards the second category, which is more anti- than pro-inflammatory. This is an advantage for the foetus but leaves pregnant women more vulnerable to certain diseases like Listeria, because her immune system can’t fight the bacteria inside cells

Altering the balance of the immune system helps but as I said there’s always both types going on in the body. Part of the problem is that an embryo must signal its presence, otherwise a woman will just begin another cycle and the pregnancy will end, an embryo needs to get noticed, but not too much and not by the wrong ‘people’. The answers to how this happen lie in the cells of the placenta, which makes sense because they are the ones in contact with the maternal tissues.

Placental cells produce a substance known as IDO, which destroys another substance that those cell killing immune cells need to function. As a result when these immune cells get to the uterus they can’t function, it’s almost like a chemical invisibility cloak around the womb. Next the placenta cells block the signal that the immune cells would use to kill them (known as FAS) and recruit other cells to effectively turn the killer cells off. The ultimate effect of all this is to reduce the numbers and effectiveness of cells that are capable of killing others cells and as soon as a woman is no longer pregnant this is all reversed. In short it is a very elegant solution to a very complex problem.

 Studying how the female body adapts to cope with pregnancy has many useful applications, most obviously in helping those women that have recurrent miscarriages because these adaptions do not happen properly. Another would be to improve the quality of life of transplant patients by finding an alternative method of preventing organ rejection than suppressing the whole immune system, which causes them to be more susceptible to infections.

I hope I have managed to explain what fascinates me about this particular subject in a way that makes other people interested and hopefully why science is such an important subject, not to mention a rewarding career. Please ask if you have any questions.  

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