T Lymphocyte Surface Antigen (CD4) (CD4)

CD4 is a glycoprotein expressed on the surface of immune cells, primarily helper T lymphocytes (T helper or Th cells), as well as some monocytes, macrophages, and dendritic cells. It serves as a co-receptor for the T cell receptor (TCR), assisting in communication with antigen-presenting cells by binding to major histocompatibility complex class II (MHC II) molecules. It plays a critical role in adaptive immunity, particularly in orchestrating the activity of other immune cells. CD4 is also the primary receptor for HIV entry into host cells.

CD4 acts as a co-receptor for the TCR during antigen recognition and signal amplification. In the context of HIV, it serves as a primary receptor for viral entry.