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

Fact 2: Antigens are proteins foreign to the body.

Given the two facts above, answer the question "what are antigens?" with the following options: 
- animals
 - negative
 - positive
 - complex
 - proteins
 - PDAs
 - humans
 - parasites


 

     proteins    