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CD4 is a glycoprotein found on the surface of certain immune cells, such as helper T cells, monocytes, macrophages, and dendritic cells. It is a monomeric type I transmembrane molecule composed of four immunoglobulin-like domains （D1 to D4） and encoded by a single gene. As a co-receptor for the T-cell receptor （TCR）, CD4 plays a crucial role in the immune response by binding to the MHC class II molecules on antigen-presenting cells, which helps T cells recognize and respond to foreign antigens. The interaction between CD4 and MHC class II molecules also activates downstream signaling pathways, leading to the activation of transcription factors and promoting T cell activation. Additionally, CD4 is significant in HIV infection, as the virus binds to CD4 to enter host cells.