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CD4, also known as cluster of differentiation 4, is a glycoprotein found on the surface of certain immune cells, particularly T helper cells. It plays a crucial role in the immune system by acting as a co-receptor for the T cell receptor （TCR） in recognizing antigens presented by major histocompatibility complex class II （MHC II） molecules. This interaction is essential for the activation of T helper cells, which then release cytokines to coordinate the immune response, including activating B cells to produce antibodies and stimulating cytotoxic T cells to destroy infected cells. CD4 is also the primary receptor targeted by HIV, leading to a significant depletion of CD4+ T cells in infected individuals, which compromises their immune system. In addition to T cells, CD4 is expressed on the surface of monocytes, macrophages, and dendritic cells, where it plays roles in cell differentiation and cytokine expression.