Peripheral blood mononuclear cells, or PBMC, are cells possessing a single, round nucleus and are collected from the peripheral, or circulating, blood. It is typically isolated from whole blood, by density centrifugation with Ficoll, a polysaccharide. They are then collected in a middle layer called the buffy coat, which comprise about 1% of the given sample and are mostly depleted of red blood cells and platelets. PBMC are a rich source of lymphocytes, such as T cells, B cells, NK cells, and monocytes, such as macrophages, dendritic cells. From this population, further cell isolations are carried out by selecting for key protein surface markers such as CD4+ (T helper cells), CD8+ (cytotoxic T cells), CD14+ (monocytes), among others. PBMC are now readily available from a number of commercial sources, with samples isolated from both healthy and diseased individuals, and can be shipped fresh or frozen.