Replicants are dangerous, so they are not allowed on Earth. If they come, they are hunted down by special cops called Blade Runners. An especially skilled one named Deckard is called out of retirement to take care of a group of replicants that have stolen a ship to come to Earth to meet their creator. Even though in the book Deckard is definitely not a replicant, everything in this movie seems to indicate he is. He even has the same red flash in his eyes when talking to Rachael, another replicant. She doesn't even know she is one, at least not until he gives her a special test to check her emotions. So if Deckard is a replicant, then he probably doesn't know he is one either.
The movie culminates in a rainy fight between Deckard and the stronger Roy. Both actors agree that Ridley Scott's assertion that Deckard is a replicant takes away from this fight. Instead of it being a fight of man over machine, it just turns into a fight between two androids. That's it. So in this sense, one could say that Deckard has to be a human. Also, it would be difficult but not impossible for Rachael to form any relationship with Deckard if she really doesn't have any emotions.