Cluster determinant 4 (CD4), a type I transmembrane glycoprotein of the immunoglobulin family of receptors, plays an integral role in signal transduction and T-cell differentiation, development and activation. CD4 is constitutively expressed on the surface of various immune cells, including monocytes, macrophages, eosinophils, dendritic cells, and most prominently T lymphocytes, where it functions as an essential co-receptor and co-ligand for T-cell receptor (TCR) and major histocompatibility complex class II (MHC-II) molecules. Ligation by MHC-II molecules on the surface of antigen-presenting cells can serve to influence adaptive immunity by facilitating helper T-cell activation and macrophage differentiation, while ligation by pro-inflammatory cytokine IL-16 can contribute to innate immunity by chemoattracting CD4-expressing peripheral immune cells along an IL-16 gradient for their recruitment and activation at sites of inflammation. The protean functionality of CD4 extends past immunity as CD4 also notably serves as the major receptor for HIV-1 and human herpes virus 7 (HHV-7) infections. During HIV pathogenesis, CD4 acts instrumentally as a high-affinity entry receptor for the internalization of HIV-1 following binding of the viral envelope glycoprotein gp120 to CD4’s extracellular domain. PeproTech’s