Could my immune system fight off an alien parasite?

Callum Campbell

We’ve all been there. You’re in outer space, the first person to explore a brand new planet bursting with alien life forms, and living your best life. Suddenly, (by accident of course) you’re sneezed on by a well-meaning but under-the-weather alien who should be home in bed. ‘Oh no!’ you think. ‘I haven’t been vaccinated against this! Is my immune system going to be able to fight it off? Am I going to die?!’

What’s an immune system and why do I need one?

Your immune system defends your body from infections. This is quite an important job when you think about it. The world around us is absolutely teeming with bacteria, viruses, fungi, parasites and other microorganisms like yeast and protozoa. From their point of view, your body is a lovely environment in which to safely live and multiply. Some of them, such as your gut bacteria, are not harmful to you. Others, however, are more selfish and just want to grow and grow and grow inside your body out of control. This latter group is harmful and can cause disease.

If your body doesn’t stay protected from these bugs all the time, you’d be quickly overrun and become very ill. This is what happens in diseases like AIDS in which crucial parts your immune system stop working. People with AIDS very rapidly get rare infections and even cancers which would ordinarily be removed without you ever knowing about it. This shows us just how important our immune system is, but how does it actually work?

How does my immune system work?

Your immune system is not an organ or body part, but rather a huge network of different types of white blood cells. These white blood cells live all across your body, not just in your blood but in your organs and tissues too. All of them do slightly different jobs and work together in a big team to protect the whole body. Scientists like to divide the immune system up into two teams: the innate and the adaptive immune system. How will these do against our alien disease?

The innate immune system

The innate immune system is like the bodyguard at the door of the body, saying who can and can’t come in. The cells of your innate immune system live in your skin and your lungs (amongst other places) because that’s where you’re most exposed to the outside world! They recognise pathogens by looking for unique molecules on their surfaces which don’t normally exist in your body. All pathogens have these, so it’s a pretty effective way to tell the difference most of the time. When it discovers a pathogen, whatever it may be, the innate immune system deals with it by engulfing it and releasing toxic chemicals. Job done, right? Well, not necessarily. Our innate immune system would definitely recognise our alien pathogen, as it hasn’t come from our own body. But that’s no guarantee it will get rid of it. Sometimes pathogens can slip through the net, which is why we get sick with Earth diseases from time to time! So what else have we got up our sleeve?

The adaptive immune system

Maybe the adaptive immune system can help. The adaptive immune system deals with pathogens that have got past the innate immune system and started an infection. Cells of the adaptive immune system live in the blood, lymph nodes and spleen. When alerted to a new infection, they pour forth into your bloodstream in huge numbers to help out wherever they’re needed.

Unlike the innate immune system, the adaptive immune system has to find out exactly which disease it’s fighting. This way it can mount a targeted attack which is more deadly to our pathogen than the one-size-fits-all approach of the innate immune system. It does this by taking a small piece of the pathogen (called an antigen), and creating its own special weapon against this target (called an antibody). And here’s the amazing part – your adaptive immune system is so clever that it can create an antibody for any antigen in the entire universe. This is why when brand new diseases appear sometimes (like COVID-19) your body can still learn how to fight it off!

So the next time you get sneezed on by an alien, don’t panic ok. Even if your innate immune system doesn’t grab it right away, you can always rely on your adaptive immune system to figure out how to get rid of it.