If a person is a chimera and has two different blood types in his veins, how does he not die? Shouldn't the immune system attack one of the blood types?

> In 1953 a human chimera was reported in the British Medical Journal. A
> woman was found to have blood containing two different blood types.
> Apparently this resulted from her twin brother's cells living in her
> body. More recently, a study found that such blood group chimerism
> is not rare.