Why does a vaccine often cause pain in your arm?

Question: Why does a vaccine often cause pain in your arm?
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Asked by ☂? to ☣ Danna, Jonny, Daniela ? on 23 Jun 2016.
- Jonathan Hunter answered on 23 Jun 2016:
  
  Well you are poking a hole in your arm with a sharp needle! The other reason is to do with the formulation that the vaccine is in. Certain salts needed to stop the vaccine going off or breaking down can cause ‘discomfort’ but it’s a toss up between some extra arm pain and the vaccine not working. I know which I would choose!
- Daniela Lobo answered on 24 Jun 2016:
  
  The piercing of skin is always a bit hurtful, either on vaccines or drawing blood.
  But the main reason that causes you to feel pain and get a bit of a swallowed arm it’s because of what vaccines actually do. The whole point of vaccines is to activate or develop an immune response – some degree of local inflammation helps recruiting immune cells to the area.

Question: Why does a vaccine often cause pain in your arm?