{"type":"root","children":[{"type":"paragraph","children":[{"type":"text","value":"CD4 is a glycoprotein expressed on the surface of T-helper cells, regulatory T-cells, monocytes, macrophages, and dendritic cells. On T-cells, CD4 is the co-receptor for the T-cell receptor (TCR). It amplifies the signal generated by the TCR by recruiting the tyrosine kinase that is essential for activating many molecules involved in the signaling cascade of an activated T-cell."}]},{"type":"paragraph","children":[{"type":"text","value":"CD4 antigen is involved in the recognition of Type II Major Histocompatibility Complex antigens (MHC-II). CD4 is also the receptor for Human Immunodeficiency Virus (HIV). It is present on most T-helper cells and normal thymocytes."}]}]}