A normally harmless substance that produces an immune response in some people is a(n) ____.

A) pathogen
B) cytokine
C) histamine
D) allergen
E) antibody

A substance that produces an immune response in some people and causes allergies is an allergen. Pathogens are disease-causing agents, cytokines are signaling molecules produced by cells, histamine is a chemical mediator released during allergic reactions, and antibodies are proteins produced by the immune system to target and neutralize pathogens.