Inflammation produced by various agents, including turpentine, killed streptococci, staphylococci, pneumococci or meningococci, and filtrates from cultures of meningococci or typhoid bacilli, produces a series of changes that are almost uniform.

Immediately after injection of the irritant there is greatly increased resistance to the production of hemorrhage by suction so that the time required may be from hour to almost 2 hours. This increased resistance to suction applied to the surface of the skin is doubtless caused by contraction of blood vessels following injection of the irritant.

The period of increased resistance is soon followed by diminished resistance of the vascular walls so that hemorrhage after 12 to 24 hours following injection occurs within from 1 to 4 minutes of suction.

The subsequent course of events varies; resistance of the vascular wall to suction, designated for convenience capillary fragility, may return to normal after from 2 to 9 days (observed with turpentine, streptococcus), or for several days may considerably exceed this level (observed with staphylococcus, pneumococcus), or may remain at a low level for a week or more (observed with tuberculin).

With inflammation in a sensitized animal (allergic inflammation) the preliminary period of resistance may be diminished and the appearance of capillary fragility hastened, so that hemorrhage occurs after 2 minutes of suction applied 6 hours after injection, and later it may fall to an even lower level (observed with hemolytic streptococci in sensitized animals).

The preparatory injection of toxic substances, such as meningococcus or typhoid filtrate, used in the production of the Shwartzman phenomenon, causes inflammation with injury of small blood vessels indicated by susceptibility to hemorrhage with suction. It is probable that subsequent intravenous injection causes hemorrhage by further injury to these injured blood vessels.