Mononuclear cells refer to blood cells that have a single, round nucleus, such as lymphocytes and monocytes. When isolated from circulating blood, they are called peripheral blood mononuclear cells (PBMC), but other sources exist, such as the umbilical cord, spleen, and bone marrow. The established method for separating mononuclear cells from blood is through density gradient centrifugation using the polysaccharide, Ficoll. Upon spinning, the cells collect in a layer called the buffy coat, which comprise about 1% of the total sample volume. From this fraction, more specific cell types can be further isolated by purification methods that target specific cell surface proteins: CD4 for T helper cells, CD8 for cytotoxic T cells, CD19 for B cells, CD14/CD16 for monocytes, among others. Mononuclear cells have been essential in the research areas of disease, therapeutics, vaccines, immunology, diagnostics and sequencing.