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).