564 IMMUNITY 



also often leucopenia due to a fall in the number of polymorpho- 

 nuclear leucocytes. These symptoms last for a few days and 

 then disappear. Such are the phenomena of the serum disease 

 after a single injection of the foreign serum. There are, however, 

 two other types of reaction described by v. Pirquet and Schick, 

 namely, the immediate and the accelerated reactions. The im- 

 mediate reaction is seen when a large dose of serum has been 

 administered and then after a certain interval of time another 

 dose of serum is injected. This interval is usually from twelve 

 days to eight weeks, though sometimes as long as six months. 

 The symptoms of the immediate reaction, which appear shortly 

 after the injection, or at least within twenty-four hours, are an 

 intense oedema locally, general exanthemata and pyrexia, though 

 the general phenomena are often little marked. The symptoms 

 pass off comparatively quickly, usually within twenty-four hours. 

 The accelerated reaction is also seen after a second injection, 

 and it may occur from six weeks up to many months after the 

 first injection. In the case of the accelerated reaction there is 

 an incubation period, but it is shorter than in the case of the 

 first injection, being usually five to seven days ; the symptoms 

 resemble those in the ordinary reaction as described above, but 

 are of rather more acute onset and last a shorter time. In the 

 interval from about the sixth week to the sixth month, there 

 may occur both the immediate reaction, and also a few days 

 later an accelerated reaction. 



The nature of the serum disease is not yet fully understood, 

 but in all probability depends upon the development of a reaction- 

 body or anti-substance, and the combination of this with a sub- 

 stance in the serum, probably an antigen, leads to the symptoms. 

 We suppose that the substances in the serum gradually disappear 

 from the body after the injection ; from about the eighth day 

 onward anti-substances appear in the blood in large amount, 

 and if antigens are still present, the combination of the two 

 brings about the phenomena described. Manifestly, if the 

 antigens have disappeared before the anti-substances appear in 

 quantity, there will be no symptoms. At a later period anti- 

 substances will be present alone in the serum, and then the 

 injection of fresh antigens brings about an immediate reaction. 

 After the anti-substances have disappeared, the injection of 

 fresh serum causes no immediate reaction, but the mechanism 

 of reaction has been stimulated by the first injection ; anti- 

 substances thus appear more quickly after the second injection, 

 hence the reaction is accelerated as compared with the reaction 

 after the first injection. 



