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Normal human and other mammalian serums contain a protein, properdin (1, 2), which is an important constituent of a natural defense mechanism of blood. Properdin, in conjunction with complement and Mg ++, participates in the destruction of certain bacteria and abnormal red cells and in the neutralization and inactivation of certain viruses. Properdin differs from antibody in many respects, particularly in its apparent lack of serological specificity, its requirements for Mg ++ and complement for its interactions, and in its physical and chemical properties.A method for the assay of properdin has been briefly described elsewhere (1). This method depends upon the requirement of properdin for the inactivation of the third component of complement (C'3) by zymosan (3). A unit of properdin is defined as the smallest amount of test sample which will reduce the C'3 titer of a properdin-deficient serum (RP) from 120 to 0 units during incubation with zymosan under standard conditions. While there are theoretical and practical objections to the zymosan assay of properdin, it has been found to be more reproducible and reliable than other types of assays now under investigation. The actual test is not difficult to perform, but careful selection and standardization of reagents are necessary.

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The attention of immunologists has long been focused on acquired immunity produced by specific antigens. In the classical sense this type of immunity to infection, which generally is of a high order, depends upon the appearance of protective antibodies. The obvious importance of such artificial immunity, and the relative ease with which these antigens and antibodies could be studied, contributed to the comparative neglect of other components of bacterial cells which may also give rise to protective reactions. Nevertheless, it has been recognized that there is also a type of resistance which transcends the limits of antigenic specificity, and from time to time this kind of protection against infection was observed even though demonstrable antibodies could not be detected. In fact it was noted by a number of workers that the injection of various substances of bacterial and of other origin produced an increase in normal resistance. While the level of resistance thus achieved was lower than that which followed the injection of specific immunizing agents, it was sufficient to indicate that it might nonetheless contribute to immunity.These observations extend back to some of the early work in microbiology and were first brought together by Kolle and Prigge (1). The more recent literature has been assembled by Brandis (2) who also reported the rapid production of a protective effect against challenge with Salmonella derby by the injection into mice of a number of bacterial vaccines or unrelated products. He called this effect "proimmunity" and considered it to be a result of non-specific stimulation of the cellular defenses of the host. Rowley (3) showed that mice injected with cell walls of Esckerichla coil, Salmonella typhimurlum, or zyrnosan (4), within the first few hours were more susceptible than normal mice to challenge with E. coll.

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