Frankly we won’t know until we find ET life. If we’re talking about something that is relatively similar to a Terran bacteria then perhaps it could interact with our cells.

Alternatively, it is probably far more likely that life from Earth would rapidly evolve to kill or replace any extant life on other worlds, especially microscopic life. We know that Terran life is tenacious. It readily thrives in hostile environments and has evolved under extreme adversarial pressures on Earth. It’s not a stretch to imagine that it could become the apex predator in an ecosystem devoid of such extreme evolutionary pressure.