In the case of Leprosy, M.leprae is the Antigen. The body's defense mechanism starts with the skin that is a good, protective covering.
Healthy skin inhibits the entry of germs except when it is cut.
The body's fluids are mildly antiseptic with tears, sweat, and gastric juices putting up a first-line of resistance to any germs that could bring on the disease. The body's protective mechanism has two aspects, Non-Specific and Specific. The macrophage then dissolves the germ by certain chemicals or enzymes. So, in the case of Lepromatous leprosy, the very cells that are meant to kill off the bacilli, are actually transporting them around the body and providing an environment in which they may even be able to multiply. At the present time though, leprosy cannot be prevented.