I am aware that theoretically, it is said that hair transplantation across two different patients (who are not identical twins) is like organ donation, and the hairs would be rejected without highly dangerous organ transplant immune system suppression medication.
But - has this actually ever been experimentally tested? For instance, taking two patients, either say father son or brothers, and then cross-transplanting a relatively small number of hairs between them (just a standard transplant, without putting them on organ transplant medication). Some number from patient A to patient B, and vice versa the same number from B to A. And seeing how many (if any) are accepted by the recipients.
Although I suspect most and perhaps all the hairs would fall out, making the procedure impractical, I think it would be an interesting experiment, and have never heard of one. I've seen a summary of an experiment that involved individuals who were not related, but not one about ones who were closely related.