The Joker infects Batman with the TITAN disease by using his blood. This implies that the blood-type of Batman is AB+, or that The Joker's blood-type is O-. Either that, or they share the same blood-type.

This is implied by Batman not showing any signs of rejecting The Joker's blood. The Joker also has a plan to infect all of Gotham City with the TITAN disease by shipping his blood to the Blood Banks that supply the city. This would further imply that The Joker is actually a Universal Doner, otherwise everyone in Gotham with a non-matching blood type would die of Hyperacute Rejection.
Contributed by Tieresias