It is all too common for people who have been in accidents of some kind to require donated blood to stay alive, but there's only so much to go around. Pluswhich, humanoid biology complicates matters: There are four major blood groups (A, B, AB, and O), and two Rhesus groups (positive and negative). People with type A blood can recieve blood from type A and O donors only; similiarly, people with type B blood can recieve type B or O blood only. A lucky few with type AB blood can recieve blood from any of the four groups, but people …