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.

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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.