Cells of the immune system are commonly purified from blood, spleen or lymph nodes. Separate cell populations (lymphocytes, granulocytes and monocyte / macrophages, erythrocytes, and cancer cells) are usually prepared by density gradient centrifugation through Ficoll-Hypaque or Percoll solutions. Separation is based on the buoyant density of each cell subpopulation at the given osmolality of the solution. Monocytes and neutrophils are also purified by selective adherence.
If known subpopulations are to be isolated, for example CD4+ or CD8+ T cells, fluorescence activated cell sorting (FACS) will be employed or magnetic beads coated with specific anti-CD4 or anti-CD8 monoclonal antibody are used. The beads are mixed with peripheral blood leukocytes and only CD4+ or CD8+ cells will bind to the beads, which are then separated out from the non-specific cells with a magnet. Another method depends on killing the undesired populations with specific antibodies and complement. In some cases, a noncytotoxic antibody or other inhibitor can block the activity of a cell subtype.
Characterization of cell types and subpopulations can be performed using markers such as specific enzymes, cell surface proteins detected by antibody binding, cell size or morphological identification.

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