It must be my boy scoutish nature but I try and carry the "Ten Essentials" whereever I go. So that includes a first-aid kit. Often no food though. However I find the thing that works the greatest in almost all situations is a nice clean and packed bandana. Clean, ointment (or booze) and wrap with the bandana. It can close a deep cut with a nice strong tie. Although now that I look at then ten essentials again it seems that I am lacking a map and compass in my bike kit. Which seems redundant on a commute hah.

Its easy to pack light with a bandana too.