Answer: An Antigen.
Any substance that triggers an immune response is known as an Antigen.
Step-by-step explanation:
An Antigen is a substance usually protein that triggers or stimulates the body to produce antibodies.
These antibodies produced by the body helps the body to fight off virus,fungi and bacteria and other harmful thing that invades the body to harm it.
Antigens are foreign to the body and therefore stimulates a response immediately the body notices it.
Antigens can also come inform of foreign bodies that trigger the body to produce antibodies for defence.