I think here's the crux. We obviously exchange some matter with interstellar space. Whether anything could survive the pulverizing impacts, the radiation, the long times involved, etc. is another question.
Most won't. The fittest will. We are the descendants of the toughest spores in the galaxy.
NASA brought back parts of an unmanned Moon probe and revitalized the bacteria found on it.
(errantly contaminated before it was sent - yeah, right...)