What does "it" refer to?

Individuals who survive an infection normally become immune to that particular disease,
although not to other diseases. The reason is that the immune system “remembers”
foreign antigens, a process called immune memory. Next time the same antigen appears, it
triggers a far swifter and more aggressive response than before.

What does the underlined "it" refer to?

Is it "the same antigen" or "the immune system"?

Thank you so much.