A surface glycoprotein that characterizes helper T cells. CD4 recognizes foreign peptides bound to MHC class II proteins expressed on the surface of B cells, macrophages, and other antigen-presenting cells. It acts as a coreceptor, binding simultaneously with the T-cell receptor to the antigen-MHC II complex and enhancing the T-cell’s response. CD4 consists of a single polypeptide chain organized into four immunoglobulin-like domains.