Cryopreserved Human Peripheral Blood Mononuclear Cells are available as positive and negative controls for T-cell monitoring in ELISPOT, ELISA, cytokine bead array, tetramer/pentamer, and flow cytometry assays. A peripheral blood mononuclear cell is defined as any blood cell with a round nucleus. These blood cells are a critical component in the immune system to fight infection and adapt to intruders. The lymphocyte population consists of CD4+ and CD8+ T cells, B cells and Natural Killer cells, CD14+ Monocytes, and Basophils/Neutrophils/Eosinophils/Dendritic cells. These cells are often extracted from whole blood or from leukopacks using ficoll, a hydrophilic polysaccharide that separates layers of blood, with monocytes and lymphocytes forming a buffy coat under a layer of plasma. Samples from each donor are tested via PCR to confirm non-reactivity.