HIV replicates in, and destroys CD4 cells that are crucial to effective immune responses. It is therefore necessary to monitor the numbers of competent cells as this is a good indicator of disease progression, patient response to therapy and what infections the patient will be prone to catching. In the laboratory cells are “counted” using flow cytometry. This technique allows scientists to label specific blood cells with molecules that will emit light of a specific wavelength when exited by a laser. Different cell populations can be counted based on the light that their specific labels emit.