We be one blood, thou and I. Or not?

We are all human beings and we are all divided into 4 blood types (except for aliens, but you are hardly going to have kids with them). In addition to your blood type, there is also an Rh factor. Most of us are Rh-positive, meaning we have a special protein in our blood. Only a quarter of the world's population lacks this protein — they have a negative Rh factor.