WHY is blood type B-negative the rarest? The fourth positive type is much more common. The fourth group in general is a mystery, because it came from the fusion of two very different types, A and B. It is a young group, and people with it have a flexible immune system. This group is unique in that it emerged not because of the influence of the external environment, but as a result of mixed marriages. It is worth mentioning that this group is the most biologically complex. ⠀ Antigens sometimes make it similar to the second, and sometimes even to the third. Occasionally, it also happens that this rare group is something like a combination of both of these groups. Thus, the fourth blood group is thought to have appeared later than all the others, about a thousand years ago, as a result of the mixing of Mongoloid and Indo-European