The finger or lobe of the ear is punctured as in making a blood count, and the blood is allowed to drop directly into normal saline, ·85 per cent. solution, containing about 1 per cent. sodium citrate; this decalcifies the blood and prevents clotting. The corpuscles are completely precipitated in the centrifuge, and then repeatedly washed with ·85 per cent. saline and centrifugated, until all traces of the citrate and serum are removed. After the final precipitation the saline solution is withdrawn, and the thin upper grayish layer of the sediment, the leucocytic cream, consisting for the most part of washed white blood cells, is removed.
III. THE SERUM.
Blood is obtained in the usual way, but is collected in small, bent, glass tubes, which can be readily held in the centrifuge and the serum separated. In obtaining normal serum, care must be exercised in selecting a healthy subject, or, what is better, obtain serum from the mixed blood of several normal persons’ “Pool.”
IV. STAIN.
With the exception of Malta fever and tubercle bacilli, Leishman’s stain, consisting of eosin and methylene-blue, in combination, or Jenner’s stain, is employed. Carbol-fuchsin is employed for the tubercle bacilli.
METHOD OF OBTAINING OPSONIC INDEX.
Special capillary pipettes, graduated into equal divisions, are required; then by means of a rubber tube or bulb equal quantities of washed corpuscles, bacterial emulsion, and patient’s serum are taken up, and all blown out upon a glass slide and thoroughly mixed. The mixture is then drawn up in a small pipette, sealed, and placed in an incubator of the ordinary type, or preferably in an opsonic incubator as proposed by Freeman; it is heated to 37° to 40° C. for fifteen minutes. A similar tube is prepared, except that the normal serum or “Pool” is employed instead of the patient’s serum. At the end of fifteen minutes the smears on microscopical slides are made from each tube, fixed, and stained. A good field is selected, one containing many leucocytes, the number of germs counted in the first fifty or hundred observed, and an average per leucocyte determined. If the average in the mixture containing the normal serum is two, we would say that the normal opsonic index is two, and if the mixture containing the patient’s blood serum is one per corpuscle, we would say that his index was one-half, etc.
Wright and his pupils, as the result of numerous observations, classify diseases due to bacterial infections:
(1) “Diseases in which the bacterial process is strictly localized” or “shut off from the lymph and blood circulations.” Furunculosis, lupus, tuberculosis, etc., in fact most chronic infections, belong to this class. “In this class the opsonic index of the blood is persistently below normal, owing to the absence of immunizing stimuli.”
(2) “Diseases in which the bacterial process is but loosely shut off, especially from the lymph circulation.” In these, usually acute, infections immunizing products from the invading bacteria from time to time get into the circulation, and the opsonic index may be normal or above or below normal. Good examples of this class are tuberculous joints, etc.