Pain Resistance: It was speculated that the Joker's exposure to chemicals may have heightened his resistance to pain. It was also implied that he was masochistic in nature, and thus enjoyed any pain received, making it an ineffective way of dealing with him.
Tainted Blood: Joker's blood was tainted. It was said that "being an avid consumer of his products, Joker's immunity to poisons has been built up over years of dedicated abuse." When a mosquito sucked the Joker's blood, it "writhes and whines, choking on tainted blood".
Toxic Immunity: The Joker was immune to his own venom as well as various similar toxins.
Cheating Death: The Joker cheated death numerous times, even in seemingly inescapable and lethal situations. He has been, among other things, caught in explosions, electrocuted, shot repeatedly, and dropped from great heights, but he always returns to once again wreak havoc.
Cosmic Awareness: The Joker has, at times, shown to be fully aware of being a comic book character, even directly talking to the reader. However, most other characters ignore this, believing it to be a manifestation of his insanity.