CD4 (or Cluster of Differentiation 4) is a glycoprotein found on the surface of immune cells. CD4 has 4 immunoglobulin (Ig) domains, a transmembrane domain and a cytoplasmic tail domain. D1 and D3 resemble Ig variable domain and D2 and D4 resemble the Ig constant domain. The D1 domain interacts with class II MHC molecules. CD4 helps T-cell receptors (TCR) interact with antigen-presenting cells. The CD4 tail interacts with tyrosine kinase Lck which activates molecular components of the signaling pathway. HIV-1 binds to CD4 initiating conformation changes in its protein gp120 and allowing the entry of HIV-1 to the host T-cell.
