Phagocytosis, with particular reference to encapsulated bacteria.

The phagocytic cells of the mammalian host provide a remarkable defense against a wide variety of bacteria, fungi, and protozoa. Whether they play a protective role in infections caused by viruses and rickettsiae is at present a matter of conjecture. The discovery of "pinocytosis" (Gey, 1954), however, and recent observations relating to bacterial endotoxins (Berthrong and Cluff, 1953; Braude et al., 1955; Collins and Wood, 1959), suggest that a mechanism closely analogous to phagocytosis may aid in the disposal of submicroscopic parasites and even of toxins of relatively small molecular dimensions. In the case of bacterial disease the process of phagocytosis per se may or may not be a critical factor in determining the ultimate fate of the parasite. Manifestly, the outcome of each encounter between microbe and phagocyte depends upon: (a) whether or not the microbe is promptly ingested and (b) whether, once ingested, it is able to survive. Most bacteria are readily ingested by both polymorphonuclear leucocytes and macrophages (Suter, 1956). Their pathogenicity is not due to resistance to phagocytosis, but rather to properties which enable them to thrive in intraphagocytic sites. These properties are as yet poorly defined (Hirsch, 1958) and will probably remain obscure until more is learned about the precise mechanisms by which phagocytic cells kill bacteria. Other papers in the present symposium discuss this highly important aspect of "cellular immunity." Although phagocytosis, per se, may not lead to the destruction of a microorganism, it may nevertheless curtail its spread. Furthermore, it may protect it against potentially lethal factors in the environment, including antibodies (Rous and Jones, 1916) and antimicrobial drugs (Magoffin and Spink, 1951; Mackaness, 1952; Shaffer et al., 1953a, b). Thus the phagocytic process may influence the course of an infection without being highly bactericidal. A number of bacterial species important to