CD4, also known as T-cell surface glycoprotein CD4, is a glycoprotein expressed on the surface of T helper cells, regulatory T cells, monocytes, macrophages, and dendritic cells. It interacts directly with MHC class II molecules on the surface of the antigen presenting cell via its extracellular domain. It is a co-receptor that assists the T cell receptor (TCR) to activate its T cell following an interaction with an antigen presenting cell. Recombinant human CD4, fused to His-tag at C-terminus, was expressed in insect cell and purified by using conventional chromatography techniques.