How does someone become allergic to something?

Question: How does someone become allergic to something?
- Keywords:
  allergen,
  body,
  immune system
Asked by edrienepadua to Christina, Colin, Jess, Samaneh, Steve on 17 Jun 2013.
- Christina Pagel answered on 17 Jun 2013:
  
  Hi,
  
  I’m afraid I don’t know! I’ve read that now that we live in a much more hygienic environment than say 200 years ago, our immune systems start overreacting to things causing an allergic reaction… but I’m not really qualified to say anything more!

Question: How does someone become allergic to something?