Here's why:

* Antigens are molecules that trigger an immune response. They are recognized by the immune system as foreign or non-self.

* The immune system is designed to identify and eliminate these foreign invaders. Antigens can be proteins, carbohydrates, lipids, or nucleic acids found on the surface of bacteria, viruses, parasites, fungi, and even some of our own cells when they become abnormal.

So, while antigens are themselves not part of the immune system, they are crucial for its functioning. They initiate the immune response, which is how the body defends itself against infection and disease.