The conclusion derived from former work, viz. Elvidge, 1928, 1930, that chemically inert suspensions of quartz and other particles intravenously administered determine a marked alteration of the white cell plasma relations, is confirmed. The particles used are graded in size, they are considered practically insoluble and no protective colloid was used in their preparation to complicate the results. Just before injection, for purposes of isotonicity, dextrose to a strength of 6 per cent is added and the suspensions are checked microscopically as should be done in all such work. The action of the particles is physical or physicochemical and not chemical in the common sense. It is a nonspecific effect since it is produced by other kinds of particles. It is non-specific also in that the resulting alteration in the white cell plasma relations, due to a change in the serum, is demonstrated by a decrease in the ability of the polymorphonuclear leucocytes to phagocyte microorganisms (fig. 3).

The serum change is due to a lack of opsonin or essential constituent thereof. This is not due to direct adsorption from, or to direct action by the particles upon the blood plasma. Neither is it due to any direct action upon the living blood leucocytes or platelets, nor to any direct or indirect effect which might be exerted through products of their disintegration.

The site of action of the particles is upon the cells of the reticulo-endothelial system (particularly of the liver, spleen and bone marrow), which cells are seen to have ingested the particles soon after injection.

The phenomena cannot be explained upon the basis of the appearance of an unknown anti-opsonin but can readily be explained as a result of inhibition or paralysing action upon the source of supply of opsonin.

It is finally concluded that opsonin production is a function of cells of the reticulo-endothelial system and that opsonin is a definite chemical substance.

It is suggested that the ultimate mechanism of action of the particles upon the reticulo-endothelial cells is due to a disturbance of cell respiration or to derangement of the metabolic activity of the phagocyting reticulo-endothelial cells brought about by the changed internal surfaces of these cells owing to the presence of the ingested particles.

From comparison of the present work with that of Jungeblut and Berlot, it is concluded, as Gordon in another way has done, that opsonin and complement are not identical substances.

It is noted that the tendency for white cells to agglutinate upon incubation is lost at the time when opsonin is reduced. This indicates a more or less simultaneous reduction in some type of agglutinin.

Previous ingestion of ink by a single leucocyte does not prevent that cell from ingesting cocci under favourable serum conditions.

Some results to show the effect of variations in the duration of the incubation period are included to indicate the extent of possible error in this regard. The effect of keeping serum at room temperature for twenty-four hours upon the deterioration of opsonin is demonstrated. A single dose may be as effective as repeated doses, if the dose is adequate. No special reactions in the phagocytosis test toward repeated doses were noted. No regular relationship could be estimated between the degree of leucocytosis, white cell reaction and degree of negative variation in the phagocytosis test except that leucocytosis exists for a time during the height and early period of the negative shift and at such time negative white cell reactions described in 1928 may be observed.

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