From what I've read, you're not totally immune from the risks of GVHD when you have a twin donor, but the risks are much much lower because you match so well. Identical twins have the same primary HLA antigens, and that's what determines a great donor match. But there are still minor antigens that differ. In fact, genetic mutations start occurring in utero, so everyone is unique at birth, even twins.
I haven't spotted any statistics on the difference in GVHD rates, but they are lowest in identical twins.
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