CD4 stands for cluster of differentiation 4. In CD nomenclature, a CD number is assigned to a cluster of monoclonal antibodies (mAbs) that recognize the same surface molecule and recognize a certain cellular differentiation pattern. CD4 refers to the group of mAbs that recognize the CD4 cell surface molecule and also refers to the CD4 molecule itself. The CD4 molecule is a glycoprotein with four extracellular domains, a transmembrane portion and a short intracellular tail. CD4 is located on the surface of various immune cells and functions in signal amplification and activation of T cells. CD4 defines the helper T cell type and is a well-known clinical cell surface protein marker for HIV progression. While CD8 positive (CD8+) T cells, also known as cytotoxic T cells, directly attack and kill virus-infected cells, CD4+ T cells prime CD8+ T cells and B cells and are also responsible for cytokine production that drives immune cell recruitment.