The human immunodeficiency virus (HIV) targets specific immune cells in the body known as macrophages because these are the cells that eliminate foreign material such as bacteria or viruses. HIV is able to reproduce and spread throughout the body if it can avoid destruction by macrophages. A recent study by Lue Dai, Ph.D., and colleagues revealed that the human cytokine IL-27 helps promote the body’s production of macrophages that are resistant to HIV.