Blood is a complex mixture of cells, proteins, metabolites and many other substances. About 56% of human blood volume is comprised of cells, more than 99% of which are erythrocytes. Human erythrocytes and thrombocytes (platelets, 0.5% blood components) do not contain nuclei. Leukocytes (0.3% of blood components), also referred to as peripheral blood mononuclear leukocytes or PBMC, do contain nuclei. Blood samples may vary widely in leukocyte (granulocytes, lymphocytes and monocytes) concentration depending on the health of the donor. Healthy blood contains less than 10E7 leukocytes per ml, while blood from an infected donor may have ten fold higher leukocyte concentrations.