Fact 1: Antigens are molecules that the immune system recognizes as foreign to the body.

Fact 2: Antigens are expressed by cancer cells.

Given the two facts above, answer the question "What can recognize cancer cells as foreign to the body?" with the following options: 
- electromagnetic energy
 - viruses
 - the immune system
 - bacteria
 - Allergies
 - Microscope
 - the colon
 - turn to a doctor


 

   the immune system      