Geographical biophysical adaptations, or mutations, have caused the indigenous people of Tribe X to produce a certain anitbody in their blood that has yet to be understood or reproduced in any medical facility.
The tribe is vehemently against the desecration of body except in homage or tribute. So, periodically, the mainland people orchestrate trades with the tribal people that enrich their lands, fortify their structures, and provide spices otherwise unobtainable to the Tribe X lands. But nothing outside of natural materials or the tribes would reject it. They do not want the pollutions of the rest of the world to interfere with their way of life, but they do realize stronger structures and certain minerals allow for survival in harsh weather or seasons where fragile crops would otherwise be threatened. So, of course, like all human existence, a little corruption comes into place and some chief somewhere decided that a fairly regular ritual to "the gods" grants their people a leeway in their beliefs such that they are no longer condemned for trading a quart of blood on a monthly basis in exchange for the goods used to enrich lands and fortify homes. You know, the things people used to do death sacrifices for except in this case you get actual evidence instead of insane beliefs.
This way your tribe remains in tact. They cannot be removed or killed because medical experts have yet to understand their mutation or whether or not it is wholly regional, regional and biological, etc. The best they can do right now is allow the tribe to continue uninterrupted and occasionally bring supplies in exchange for the next batch of antibody rich blood.
There's plenty of historical records of human structures having only one primary export. In this case, it's blood. But that doesn't mean there's no demand for tribal arts and housewares. Maybe they make a mean cooking kiln too.
Eventually medicine may have all the answers and realize they dont need the tribe anymore or just outright kill them and take their land when the answers are found. But hey, nothing new right?