The phagocytic activity of exudative rabbit macrophages and polymorphonuclear leucocytes was compared by a quantitative method in vitro. The work reported demonstrated the following relations when these two types of cells were afforded an equal opportunity for contact with various kinds of particulate object.

In saline solution there was little or no phagocytosis by either kind of cell when certain acid-fast bacilli, sheep erythrocytes, Bact. coli or B. subtilis were used as test objects. Various strains of virulent mammalian tubercle bacilli, however, were engulfed readily and with no significant difference by both types of cells.

Untreated or protein-treated collodion particles of small size were, when suspended in saline solution, much more readily engulfed by macrophages than by polymorphonuclears. These were the only test particles toward which the two types of phagocytes exhibited a constant and significant difference in behavior.

Phagocytosis by both types of cells was promoted to a parallel degree by freshly drawn normal serum, as shown in the experiments with amphibian and avian tubercle bacilli. Inactivation of the serum destroyed the opsonic effect with respect to both types of phagocytes.

Interaction of test particles with homologous immune serum promoted phagocytosis by both macrophages and polymorphonuclears. This has invariably been found to be the case with all the test particles. The enhancement of phagocytosis by both types of cells has been of approximately parallel degree with the amphibian, avian, and avirulent bovine strains, with three virulent mammalian strains of tubercle bacilli, and with the colon bacillus. With other virulent strains of mammalian tubercle bacilli and with protein-treated collodion particles the initial high degrees of spontaneous phagocytosis have tended to obscure the parallelism. In the case of erythrocytes and B. subtilis the polymorphonuclears have exhibited a somewhat greater response than the macrophages to the phagocytosis-promoting effects of immune serum.

That the bacteriotropic effects of immune sera are exerted essentially upon the particle rather than upon the phagocytic cell has been shown to be true for both types of cells, using amphibian and avian tubercle bacilli, protein-treated collodion particles and B. subtilis.

The specific phagocytosis-promoting components of immune serum for both kinds of cells are distributed between the globulin fractions. This has been shown with an anti-cheloneic, and anti-edestin immune serum.

The phagocytic response to tropin-treated erythrocytes, hay bacilli, and protein-coated collodion particles has been found to be equally specific for both kinds of phagocytes. While phagocytosis by both macrophages and polymorphonuclears was greatly enhanced by homologous immune sera, or their globulin fractions, neither type of phagocyte has ingested these test-objects to the same extent when heterologous have been substituted for homologous sera. With two strains of acid-fast bacteria other than virulent mammalian tubercle bacilli, cross reactions with immune sera were observed, but both types of cells responded by engulfing the bacteria to an equal degree.

The nearly complete agreement between the phagocytic behavior of macrophages and that of polymorphonuclear leucocytes was also demonstrated when these cells were tested under conditions which tended to produce prozone effects, as was the case in some experiments with various acid-fast bacteria, erythrocytes and B. subtilis.

The mechanism of bacteriotropic action has been studied by various tests which demonstrate changes of surface properties produced by interaction of sensitizing serum and antigen particle. The previously reported close correlation between surface changes and promotion of phagocytosis by sensitization with immune serum or its globulin fractions has been confirmed for acid-fast bacteria and for protein-treated collodion particles, and extended, with the qualifications noted in the text, to virulent mammalian tubercle bacilli and sheep erythrocytes.

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