Enumeration of human lymphocyte subsets by monoclonal antibodies and flow cytometry: a comparative study using whole blood or mononuclear cells separated by density gradient centrifugation.

Enumeration of lymphocyte subpopulations by combined use of flow cytometry and monoclonal antibodies is influenced by the separation method used. T or B lymphocyte antigen frequencies do not differ in samples of whole blood or after separation on Ficoll-Paque or Percoll, but there is a significant increase of Leu+ Leu11+ lymphocytes showing strong natural killer activity at the sample-separation medium interface. At the bottom of the tubes a selective loss of OKT+, Leu7+, Leu11- cells is found; at this level cytotoxicity is very low. Our data suggest that Leu7+ cells could be subdivided into 2 subpopulations differing in reactivity with the monoclonal antibody to Leu11, natural killer activity and density. Differences observed in the percentages of Leu7+, Leu11+ lymphocytes between whole blood and separated cells could be due to an enrichment produced by centrifugation or to the presence in whole blood of a factor interfering with antibody binding.