Provide your best answer for the following question. Give ONLY the answer, no other words or explanation.
For example:

Answer: <most likely answer, as short as possible; not a complete sentence, just the answer!>.

The question is: Roger visited his grandfather who lives on a farm. While he was there, he helped his grandfather move hay out of the barn. As Roger worked in the barn he began sneezing. Which system of the body most likely caused Roger to sneeze?
immune
Nervous/Respiratory.