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CD4 （cluster of differentiation 4） is a glycoprotein found on the surface of immune cells such as T helper cells, monocytes, macrophages, and dendritic cells. It is a type of white blood cell that helps fight infection by triggering your immune system to destroy viruses, bacteria, and other germs that may make you sick. CD4 is also a receptor for the HIV virus, and when the virus infects cells with CD4 surface proteins, it depletes the number of T cells, B cells, natural killer cells, and monocytes in the patient's blood.