Here's why:

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

* Microbes (bacteria, viruses, fungi, parasites) have unique antigens on their surfaces that distinguish them from human cells. These antigens are what the immune system targets when fighting infections.

* Foreign tissue (like a transplanted organ) also contains antigens that are different from the recipient's own antigens. This is why organ transplants require immunosuppressant drugs to prevent rejection.

* Cancerous cells often express abnormal or altered antigens. These "tumor antigens" can help the immune system recognize and destroy cancerous cells.

In summary: Antigens are the key to the immune system's ability to identify and attack harmful invaders and abnormal cells.