Retroviruses replicate inside cells they have invaded, using an enzyme called "reverse transcriptase" to transcribe RNA into DNA. In this way they can evade the body's natural immune defense mechanisms as they make new copies of themselves. The most important retrovirus is the human immunodefeciency virus (HIV). HIV invades and destroys host cells, particularly T-helper lymphocytes, which are crucially important in maintaining the body's immune defense mechanisms. Disruption of immune defense mechanisms following the destruction of T-helper lymphocytes is the main way in which HIV leads to AIDS.