A group of protein markers on the surface of a white blood cell. These markers are used to classify immune cell types and establish international nomenclature standards. Although found on many blood cells and some nonblood cells, they are used most often to refer to lymphocytes. Markers for CD3 are found on all mature T cells in association with T-cell antigen receptors; CD2 and CD7 markers are found on immature T cells. Markers for CD4 are found on all T helper cells, macrophages, and some B cells. Markers for CD8 identify cytotoxic T cells, which are essential to the defense against viral infections. Each marker has a specific function in the cell, such as passing a signal from the T-cell receptor to the cytoplasm.