Immune system and infections
The immune system is made up of a whole range of different cells that can do various things to help protect the body from almost all germs. Some immune cells give orders, while others fight off diseases or collect the body's garbage. But if the immune system is too aggressive and, for instance, fights a false enemy, it can cause problems like allergies. If it begins to attack the body itself, like in auto-immune diseases such as arthritis, it can damage nerves and hormone-producing glands, and lead to stiff joints.