Read the following paragraph and answer the question. Give ONLY the answer, no other words or explanation.

For example:

Answer: <most likely answer, as short as possible>.

Paragraph: 
Did you ever hear of hay fever? It’s not really a fever at all. It’s an allergy to plant pollens. People with this type of allergy have symptoms such as watery eyes, sneezing, and a runny nose. A common cause of hay fever is the pollen of ragweed. Many people are also allergic to poison ivy ( Figure below ). Skin contact with poison ivy leads to an itchy rash in people who are allergic to the plant.


Question: Hay fever is actually an allergy to what substance, vital to plant reproduction?
pollen
Pollen.