Detects threats: The immune system looks for foreign substances or unhealthy cells. 

Sends signals: When a threat is detected, the immune system sends signals to other cells to attack. 

Destroys invaders: The immune system attacks and destroys microbes, like bacteria, viruses, and fungi, or unhealthy cells, like cancer cells. 

Repairs damage: The immune system heals the damage caused by invaders. 

Remembers threats: The immune system keeps a record of every microbe it has defeated so it can quickly recognize and destroy it again.