Researchers are investigating a range of ways to restore aspects of the aged immune system to youthful levels. Here is one: "By comparing the immune responses of both, young and old mice, to bacterial infection they found that the number of macrophages, one of the major cell populations involved in the elimination of infecting bacteria, decreases rapidly in aged mice. This decline in the number of fighters and the associated weakness of the immune defense may be responsible for the age-associated increase in susceptibility to infections. [Researchers] have succeeded to enhance the resistance to an infection in aged mice by treating them with a macrophage-specific growth factor. This treatment increases the amount of macrophages in aged mice and improves their capacity to fight the infection. ... The treatment made aged mice much more resistant and they could fight much better the infection. The results of our study indicate that repeated prophylactic administration of this growth factor can help to maintain the macrophage compartment in the elderly and the fitness of the immune system."