CD4 is a protein found on the surface of certain immune cells, primarily T helper cells （also known as CD4+ T cells）. CD4 stands for "cluster of differentiation 4" and is a glycoprotein that serves as a receptor for major histocompatibility complex （MHC） class II molecules, which are present on the surface of antigen-presenting cells （APCs） such as macrophages, dendritic cells, and B cells. The primary function of CD4+ T cells is to orchestrate and regulate the immune response. They are crucial in activating other immune cells, including macrophages, cytotoxic T cells （CD8+ T cells）, and B cells, to eliminate pathogens and infected cells. CD4+ T cells can be further classified into different subsets based on their function and the cytokines they produce, such as Th1, Th2, Th17, and regulatory T cells （Tregs）. CD4 is also used as a marker for identifying and quantifying T helper cells in clinical settings. Decreased CD4 counts are often associated with immune suppression, such as in HIV/AIDS, where the virus specifically targets and destroys CD4+ T cells, leading to a decline in immune function. Monitoring CD4 counts is important in managing HIV/AIDS and assessing the immune status of patients.