Allergic to just one part of an animal?

Can you be allergic to one part of an animal but not another? What's the point of dreaming? What happened before the Big Bang? What's in wine that causes hangovers, and is this an allergic reaction? Are we seeing into the past when we detect light from far away in space? Why does driving through a long tunnel or across a bridge make me feel giddy? Lester and Dr Chris make your head spin with the answers!