The zipper on your jacket has two sets of interlocking teeth that have to fit together perfectly in order for the zipper to work properly. If the match is imperfect then your jacket is pretty useless. Similarly, each copy of the viral DNA must match up correctly in order for it to function. The enzyme that HIV uses late in its life cycle to cut HIV proteins into small functional proteins so that they can be incorporated into new viruses. Protease is a general term for any enzyme that can cut up proteins.
Defective viruses can bud from the cell but hey have lost their infectivity.
Normally these cells act to gobble up infectious microbes in the blood and tissues of the body and destroy them before they have a chance to cause infection. Certain bugs like HIV have figured out a way to live inside macrophages without being killed. Without these cells the immune system can not communicate effectively and loses the ability to fight off infections that it can normally control. The CD4 marker is like a doorknob that HIV must turn before it can gain entry into its new home. Recent evidence indicates that the initial infecting strain of HIV targets macrophages first .