Why is the immune system able to fight off some viruses but not others, leading to chronic, life-threatening infections like HIV and hepatitis C?

 

A new UCLA AIDS Institute study suggests the answer lies in a protein called interleukin-21 (IL-21), a powerful molecule released by immune cells during chronic infection. Published May 7 in the online edition of Science, the finding could explain how the immune system limits viral replication, restricting a...