“CD” refers to a class of molecules found on the surface of leucocytes (white blood cells). They are receptors that receive information enabling these cells to interact with each other. There are many types of CD receptors, each having a distinct function. They are classified with numbers that identify their function and distinguish different cell types. A cell that bears a particular type of CD is represented with a plus sign (e.g. CD8+ cell), however often the plus sign is dropped. The most commonly known and discussed CD receptor in HIV infection is CD4.