The large measure of success attendant upon the use of immune sera in the treatment of certain types of pneumococcus pneumonia has given rise to the hope that other infections of similar origin might be benefited.

The invasion of the pleural cavity by the pneumococcus, secondarily to pneumonia, is common—in fact, it is the rule rather than the exception. Whether this infection gives rise to dry pleurisy, or empyema, depends upon the virulence of the micro-organism, the amount of chemiotactic action exerted and the destructive action of elaborated toxins. The very nature of the lymphatic circulation of the pleura with the opportunities for the inflow of immune substances, unquestionably tends to reduce materially the number of secondary cases of empyema. Where, however, the lymph channels are blocked with fibrin and cells as the result of acute pleural infection, there may be a great difference between the amount of circulating antibodies in the body at large and their abundance in the pleural exudation.

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