Why You’re Not Dead Yet – A Crash Course on Fighting the Flu

You’re sitting at home, watching TV, and then, suddenly, you feel it. Did your throat always feel so dry? Was your nose that stuffed up this morning? Then, you realize it. You heard someone cough near you on the train yesterday morning, and now, you’re one of the 60 million Americans each year who’ve caught the flu. 

You forgot to get your flu vaccine, so your body is unprepared. A vaccine activates your defenses and leaves you ready to defend yourself, so, without one, your body is left scrambling to respond. The flu is a particularly nasty invader, as each year it changes to try and sneak past your defenses. This means you can’t rely on your body’s memory from last year’s flu to protect you this year.

Despite 60 million Americans catching the flu each year, less than one percent of them will pass away because of it. This high survival rate is because of our immune systems. 

Your immune system has two basic forms, adaptive and innate. Once a germ like the flu, known as a pathogen, gets past your skin, it first has to contend with your innate immune system. This side of your immune system is called ‘innate’ because it’s ingrained in your DNA, so your innate system will be about the same as your friend’s. 

If you think of your body like a castle, you can start to make sense of the other half of your immune system: your adaptive immune system. First, your skin is the moat around the castle – a good defense, but not perfect, as invaders can get around the moat or cross it. Your innate immune system is then the wall behind the moat. It’s harder to break past than the moat, but also not impossible. The final problem for the invaders is going to be the group of fighters behind the wall – your adaptive immune system. The flu invaders will have to deal with the moat and wall first, but the real battle will be with the fighters that can adjust — or adapt — their strategies to beat them.

By getting the flu vaccine, you warn your fighters of what’s coming, and they get ready to fight the flu off as soon as it tries to sneak in. But when you forget your flu shot, your fighters don’t get this warning. They then have to go through a long list of steps to fight off the flu, leaving you sick for longer. This long number of steps will start with the innate system.

The innate system works by looking for specific pieces that are common to most invaders. When different parts of your innate immune system recognize a piece of the flu virus that snuck inside, they’ll attack it. The innate system doesn’t know that the little piece they found is the flu, it just knows it’s not you, so it wants to get rid of it. Because the innate system doesn’t care what outside thing it’s attacking, it can act quickly to fight off the invaders.

But while the innate system works, it’s not always enough. If it were, that sneeze on the subway wouldn’t knock you out for a week, and it wouldn’t be so important to get your flu shot every year. You need the adaptive system because certain bugs are sneaky and can plan ways around the castle wall. These are the flu invaders. But once they’ve breached the wall, they have to fight with the army inside the castle. 

Just like a soldier may change how they fight based on the enemy they’re up against, the adaptive immune system changes for each different invading bug. But to do this, the adaptive immune system is going to need time. The castle wall is always ready, but the soldiers behind it need to figure out if they’re going to need a battle axe or a sword to beat the invader. By vaccinating, you tell your adaptive immune fighters which weapon to have ready, instead of having them scramble around while you lay in bed sick. 

While the innate immune system is the same for everyone, the adaptive immune system is not. 

Consider again, the flu on the train. One person with the flu may only have a small headache or a sore throat. But someone else, and hopefully not you, may be bedridden for a week. This isn’t because the virus has changed drastically from the time it was passed from the first to the second person. It’s because the second person’s adaptive immune system cells weren’t quite as ready to fight the flu.

Your adaptive immune system has three main fighters: killer T cells, helper T cells, and B cells. B cells are like archers, shooting arrows at an invader from a distance. These arrows are called antibodies, which are proteins that can bind and stop invaders. Your next fighters are Killer T cells. These are the sword fighters that kill infected cells and stop the flu virus from spreading within you. Your last fighter will be your helper T cells – the squires that help support the archers and sword fighters. 

The helper T cells may seem less important because they’re not at the front lines, but they’re necessary if you want to fully beat the flu. This is because they release special chemical signals to tell your adaptive fighters how to change to beat the invader. But these signals have a side effect – they’re the reason you feel awful when you’re sick. They cause fevers because if your body heats up, the invader can’t survive. They also cause you to swell up, because this brings more fighters to an infection. 

The adaptive immune response, whether it’s from vaccines or from fighting a disease from scratch, creates immune memory. This is just like how an army will learn how to fight a specific invader once and can use that strategy again. Once you have fought off a specific type of flu once, some of your adaptive fighters will stick around to be ready in case it tries to invade again. 

Because these fighters will already be around, you won’t need to go through all the steps with the innate and adaptive system to fight off that specific flu again. This means you won’t feel sick at all, or will only feel a little off. But, because the flu changes each year, you can’t rely on last year’s vaccine to protect you this year. The flu knows your fighters are ready from last year, and can change to try and outsmart them. This is why the flu shot is so important – it prepares you each year for the flu’s next disguise.

To protect you, and those around you, from this sneaky invader, get your flu memory going in the fall, before the flu hits. Find your closest flu shot clinic here.

While you’re here, check out what’s in the flu vaccine and how safe and effective it is, and read as an expert answers some common questions about the flu shot.

Anja’s article is part of a collaboration between the Illinois Science Council and the University of Chicago.