Walk through a pharmacy, and you're likely to see plenty of products claiming to boost your immune system. There are many problems with those claims. Despite the way it's portrayed, the immune system is not one thing: it's a complex system that incorporates multiple cells, organs, and biological functions. In the simplest terms, the immune system is made up of two parts: innate and acquired. The innate response is what you notice when you get sick. It attacks infection with mucus, fever, coughing, and, in general, inflammation. What it lacks in specificity, it makes up for in speed.
But it doesn't actually drive infection out of the body. That's up to the acquired response: the antibodies your system collects from past illnesses and vaccines. (In fact, vaccinations could be considered the exception to the immunity rule—they're the only known way to improve your immune system. Even still, a vaccine can't boost the system's overall response; just its ability to fight the bug it's designed for). Since over-the-counter immunity boosters aren't vaccines, they must claim to boost the innate response—that is, the one that can't actually drive away infection but does cause inflammation, giving you inconvenient symptoms like a runny nose and fever. Even if you could boost your innate immune system and suffered through those symptoms, it wouldn't be good for you. Long-term inflammation is associated with all sorts of ill health effects, including a hardening of the arteries and heart attacks.
You can, however, put your body in the best possible shape for the next time infection strikes. Eat a balanced diet, get enough rest, drink enough water, and exercise, and your plain old garden-variety immune system will be ready to fight off invaders. And if you do get sick, opt for a little bit of immune suppression: that's how ibuprofen and antihistamines make you feel better, after all. Dive into the vast complexity of the immune system with the videos below.