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CD4 is a glycoprotein found on the surface of immune cells, primarily known for its role as a coreceptor on T-helper cells, a subset of T lymphocytes. CD4 is used as a marker to distinguish between different T cell subsets, particularly T-helper cells （CD4+ T cells） and T-regulatory cells （also known as Tregs, which can be CD4+CD25+）. CD4 acts as a coreceptor with the T cell receptor （TCR） to recognize antigens presented on the surface of antigen-presenting cells （APCs） by MHC class II molecules. This interaction is crucial for the activation of T-helper cells. CD4+ T cells are central to the regulation of immune responses. They help activate B cells, macrophages, and other immune cells, and they produce cytokines that influence the type of immune response （e.g., TH1, TH2, Tfh, Tregs）.