There are many other parts of the immune system that are just as obscure, so let's start by learning about all of the parts. The most obvious part of the immune system is what you can see. For example, skin is an important part of the immune system. It acts as a primary boundary between germs and your body. Part of your skin's job is to act as a barrier in much the same way we use plastic wrap to protect food.
Skin is tough and generally impermeable to bacteria and viruses.
The skin also secretes antibacterial substances. These substances explain why you don't wake up in the morning with a layer of mold growing on your skin - most bacteria and spores that land on the skin die quickly. Your nose, mouth and eyes are also obvious entry points for germs. Saliva is also anti-bacterial. Since the nasal passage and lungs are coated in mucus, many germs not killed immediately are trapped in the mucus and soon swallowed.