CD4, also known as cluster of differentiation 4, is a glycoprotein expressed on the surface of certain immune cells, particularly helper T cells. It plays a crucial role in the immune system by acting as a co-receptor with the T cell receptor (TCR) to bind to major histocompatibility complex class II (MHC II) molecules on antigen-presenting cells, facilitating the activation and differentiation of T cells. CD4+ T cells, often referred to as helper T cells, are essential for coordinating immune responses. They secrete various cytokines to activate other immune cells like B cells, macrophages, and cytotoxic T cells, and can differentiate into subtypes such as Th1, Th2, Th17, and regulatory T cells, each with specific functions. Additionally, CD4 is significant in medical contexts like HIV infection, where the virus targets CD4+ T cells, leading to a decline in their numbers and compromising immune function.