We encounter billions of germs every day through the air we breathe, the food we eat and the things we touch. Considering all of the potential threats to your body, including viruses, bacteria, fungi, toxins, and parasites, it's amazing that most of them have little or no impact on your health and well-being. We enjoy relative safety from these threats thanks to the complex arsenal of defenses that make up the human immune system.
You can get a sense of how much the immune system does for you by reflecting on what happens when it stops working. For example, the reason the HIV virus is so dangerous is because it destroys immune cells and stops the immune system from responding to new infections. HIV patients are likely to have repeated sinus, lung and gastrointestinal infections. Also, patients undergoing chemotherapy often have low white blood cell counts, and must take extreme care not to get bacterial infections of any kind. When the immune system is not working, even the most common infection can be lethal.
Given all of the work that it does to protect the body from various attackers, it would be an understatement to say that the immune system is complex. There are many organs involved, several distinct systems working together, millions of different types of immune related cells playing a role, and countless paths for communication and coordination traveled throughout the process. The immune system is fascinating not only because of how each of these parts works, but also because of the complex integration and control required to keep this crucial system in balance.