I'm still on season one (yeah, yeah... lots of catching up to do), and I think it was episode 8 that I was watching (maybe 9) tonight, when we had one of the flashbacks.
As a child, Dexter was injured while playing. He needed a blood transfusion, but he has the rarest of blood types: AB-. The hospital was stymied by the lack of a suitable donor, and only Dexter's foster father was able to identify a blood relative who would have the right blood type.
- It's not that hard to find a donor for a recipient with type AB-. Any Rh-negative donor will do, because AB types can also receive A, B, or O types without any problems.
- The episode implies that the donor is Dexter's biological father, but if Dexter is type AB-, then his father must be either A- or B-, not AB-, so even if Dexter needs an exact match for some reason, his biological father couldn't be an exact match.
Edit: I suppose Dexter's biological father could be AB-, since he could donate either the A or the B if his wife donated the other one. My bad. Point #1 still stands.
Further Edit: A little research indicates that less than 10% of the population of Florida has Rh-negative blood, so finding a relative might still be fairly important.