The immune system is a complicated and intricate system that contains specialized cells and tissues that protect us from external invaders and our own altered cells. Which term is used to define any substances capable of inducing a specific immune response and of reacting with the products of that response?

An Antigen.

Any substance that triggers an immune response is known as an Antigen.

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.

Explanation:

If youre here for study island, the answer is "an invasion of a microorganism from the external environment"

the missing space is phylum