The Joker has injected himself with a substance that is slowly broken down by the body and secreted. In the presence of this substance, the explosive chemicals cannot be triggered. Only a small amount is neccessary to keep them in their inert state, an ammount small enough to be gained by simply being in the same room or coming in physical contact with someone who is secreting the substance. So as long as the victims continue to meet Mr. Longreen on a regular basis, they remain safe. But if they ever go past a certain window, the chemicals are “triggered”, intiating a fatal explosion. At that point even coming in contact with a secreter isn’t enough to stop the reaction. But a large dose of the substance will counteract it.

Batman, with his not very modest detective skills, had already deduced that this secreting stabilizer was the only mechanism by which the Joker could possibly be causing the explosives. So he knew that the Joker would have the antidote “on hand”. And knowing that Alfred was going to explode any second, resorted to the most expedient method for getting it out of the Joker and into Alfred.

And probably gave his a complete transfusion of synthetic blood afterwards to clean him out. (What was that? Why not do that first? Sorry old chum, I am too modest to explain…)

And why am I so sure that Grant Morrison has read this comic book?

In that tale, the Joker captures various people who have crossed him and ties to giant explosive candles atop a giant birthday cake. One of the people he captures is Alfred Pennyworth, and this story is the only reason I can think of that he would want to take revenge on Bruce Wayne’s butler.