Q: Given a scientific passage and an answer, generate a question for the given answer.
Passage: Figure 42.10 An antigen is a macromolecule that reacts with components of the immune system. A given antigen may contain several motifs that are recognized by immune cells. Each motif is an epitope. In this figure, the entire structure is an antigen, and the orange, salmon and green components projecting from it represent potential epitopes. Answer: immune system
A:
An antigen is a macromolecule that reacts with components of what?