J. H. Humphrey

Expression of an immune response is the net result of complex synergis­ tic and antagonistic activities performed by a variety of cell types. It includes macrophages, T and B populations which may interact in performance of a response, and suppressor cells interfering with it. Accordingly, a lack of res­ ponse may not necessarily indicate absence of immunocompetent cells, but rather nonexpression of competence. Thus, one should consider two possible situations, which are by no means mutually exclusive, to account for immuno­ logic unresponsiveness: (a) one or more of the cell populations composing the synergistic unit is absent or immature, and (b) an antagonistic unit which interferes with the response is dominating. In view of this, an approach to development of immune reactivity necessitates parallel surveys of development of cells with the potential to perform, as well as of cells which can suppress the response. Classification of the various cell types has been based so far on their phenotypic properties (e. g. , membrane antigen markers, cell receptors, pro­ duction and secretion of immunoglobulins, etc. ). Genotypically, T and B cells may represent either separate, independent cell lines, or different stages of development within the same cell lineage.

Prominent progress in molecular biology was only made when it became possible to separate functionally distinct molecules by taking advantage of their biophysical properties. Likewise, the analysis of the functions of hetero­ geneous populations of immunocompetent cells, as to the functional properties of their various subpopulations, can not be done until these can be isolated in reasonably pure form by selective fractionation. During the last few years significant advances have been made in this field, and cells have been separated according to size, density or charge (MILLER et aI. , 1969; SHORTMAN, 1968; ANDERSSON, 1973 c), or by taking advantage of more specific surface markers to allow selective depletion or enrichment of a given subpopulation of cells (WIGZELL and ANDERSSON, 1971). Although separation techniques have been used in a variety of cellular systems, they have been particularly useful in the study of reticuloendothelial cells and primarily in the study of cells partici­ pating in the immune responses. Quite extensive reviews have been written which well cover the methods used for separation of cells and the results obtained with the various approaches (WIGZELL and ANDERSSON, 1971; SHORTMAN, 1972). To review this work is becoming a more and more voluminous task. As data rapidly accumulate, we will not try to make such a complete review.